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Array and object iteration in JavaScript? [duplicate] - javascript

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}

Related

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}

I have a JavaScript object like the following:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
How do I loop through all of p's elements (p1, p2, p3...) and get their keys and values?

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.
Here is the snippet:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " -> " + p[key]);
}
}
For-of with Object.keys() alternative:
var p = {
0: "value1",
"b": "value2",
key: "value3"
};
for (var key of Object.keys(p)) {
console.log(key + " -> " + p[key])
}
Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties
Using the new Object.entries() method:
Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.
const p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (const [key, value] of Object.entries(p)) {
console.log(`${key}: ${value}`);
}

Under ECMAScript 5, you can combine Object.keys() and Array.prototype.forEach():
var obj = { first: "John", last: "Doe" };
Object.keys(obj).forEach(function(key) {
console.log(key, obj[key]);
});
ECMAScript 6 adds for...of:
for (const key of Object.keys(obj)) {
console.log(key, obj[key]);
}
ECMAScript 8 adds Object.entries() which avoids having to look up each value in the original object:
Object.entries(obj).forEach(
([key, value]) => console.log(key, value)
);
You can combine for...of, destructuring, and Object.entries:
for (const [key, value] of Object.entries(obj)) {
console.log(key, value);
}
Both Object.keys() and Object.entries() iterate properties in the same order as a for...in loop but ignore the prototype chain. Only the object's own enumerable properties are iterated.

You have to use the for-in loop
But be very careful when using this kind of loop, because this will loop all the properties along the prototype chain.
Therefore, when using for-in loops, always make use of the hasOwnProperty method to determine if the current property in iteration is really a property of the object you're checking on:
for (var prop in p) {
if (!p.hasOwnProperty(prop)) {
//The current property is not a direct property of p
continue;
}
//Do your logic with the property here
}

The question won't be complete if we don't mention about alternative methods for looping through objects.
Nowadays many well known JavaScript libraries provide their own methods for iterating over collections, i.e. over arrays, objects, and array-like objects. These methods are convenient to use and are entirely compatible with any browser.
If you work with jQuery, you may use jQuery.each() method. It can be used to seamlessly iterate over both objects and arrays:
$.each(obj, function(key, value) {
console.log(key, value);
});
In Underscore.js you can find method _.each(), which iterates over a list of elements, yielding each in turn to a supplied function (pay attention to the order of arguments in iteratee function!):
_.each(obj, function(value, key) {
console.log(key, value);
});
Lo-Dash provides several methods for iterating over object properties. Basic _.forEach() (or it's alias _.each()) is useful for looping through both objects and arrays, however (!) objects with length property are treated like arrays, and to avoid this behavior it is suggested to use _.forIn() and _.forOwn() methods (these also have value argument coming first):
_.forIn(obj, function(value, key) {
console.log(key, value);
});
_.forIn() iterates over own and inherited enumerable properties of an object, while _.forOwn() iterates only over own properties of an object (basically checking against hasOwnProperty function). For simple objects and object literals any of these methods will work fine.
Generally all described methods have the same behaviour with any supplied objects. Besides using native for..in loop will usually be faster than any abstraction, such as jQuery.each(), these methods are considerably easier to use, require less coding and provide better error handling.

Preface:
Object properties can be own (the property is on the object itself) or inherited (not on the object itself, on one of its prototypes).
Object properties can be enumerable or non-enumerable. Non-enumerable properties are left out of lots of property enumerations/arrays.
Property names can be strings or Symbols. Properties whose names are Symbols are left out of lots of property enumerations/arrays.
Here in 2018, your options for looping through an object's properties are (some examples follow the list):
for-in [MDN, spec] — A loop structure that loops through the names of an object's enumerable properties, including inherited ones, whose names are strings
Object.keys [MDN, spec] — A function providing an array of the names of an object's own, enumerable properties whose names are strings.
Object.values [MDN, spec] — A function providing an array of the values of an object's own, enumerable properties.
Object.entries [MDN, spec] — A function providing an array of the names and values of an object's own, enumerable properties (each entry in the array is a [name, value] array).
Object.getOwnPropertyNames [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are strings.
Object.getOwnPropertySymbols [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones) whose names are Symbols.
Reflect.ownKeys [MDN, spec] — A function providing an array of the names of an object's own properties (even non-enumerable ones), whether those names are strings or Symbols.
If you want all of an object's properties, including non-enumerable inherited ones, you need to use a loop and Object.getPrototypeOf [MDN, spec] and use Object.getOwnPropertyNames, Object.getOwnPropertySymbols, or Reflect.ownKeys on each object in the prototype chain (example at the bottom of this answer).
With all of them except for-in, you'd use some kind of looping construct on the array (for, for-of, forEach, etc.).
Examples:
for-in:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name in o) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.keys (with a for-of loop, but you can use any looping construct):
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.keys(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.values:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const value of Object.values(o)) {
console.log(`${value}`);
}
Object.entries:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const [name, value] of Object.entries(o)) {
console.log(`${name} = ${value}`);
}
Object.getOwnPropertyNames:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertyNames(o)) {
const value = o[name];
console.log(`${name} = ${value}`);
}
Object.getOwnPropertySymbols:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Object.getOwnPropertySymbols(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
Reflect.ownKeys:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (const name of Reflect.ownKeys(o)) {
const value = o[name];
console.log(`${String(name)} = ${value}`);
}
All properties, including inherited non-enumerable ones:
// A prototype object to inherit from, with a string-named property
const p = {answer: 42};
// The object we'll look at, which inherits from `p`
const o = Object.create(p);
// A string-named property
o.question = "Life, the Universe, and Everything";
// A symbol-named property
o[Symbol("author")] = "Douglas Adams";
for (let depth = 0, current = o; current; ++depth, current = Object.getPrototypeOf(current)) {
for (const name of Reflect.ownKeys(current)) {
const value = o[name];
console.log(`[${depth}] ${String(name)} = ${String(value)}`);
}
}
.as-console-wrapper {
max-height: 100% !important;
}

You can just iterate over it like:
for (var key in p) {
alert(p[key]);
}
Note that key will not take on the value of the property, it's just an index value.

In ECMAScript 5 you have new approach in iteration fields of literal - Object.keys
More information you can see on MDN
My choice is below as a faster solution in current versions of browsers (Chrome30, IE10, FF25)
var keys = Object.keys(p),
len = keys.length,
i = 0,
prop,
value;
while (i < len) {
prop = keys[i];
value = p[prop];
i += 1;
}
You can compare performance of this approach with different implementations on jsperf.com:
Extend Implementations
Object keys iteration
object literal iteration
Browser support you can see on Kangax's compat table
For old browser you have simple and full polyfill
UPD:
performance comparison for all most popular cases in this question on perfjs.info:
object literal iteration

Performance
Today 2020.03.06 I perform tests of chosen solutions on Chrome v80.0, Safari v13.0.5 and Firefox 73.0.1 on MacOs High Sierra v10.13.6
Conclusions
solutions based on for-in (A,B) are fast (or fastest) for all browsers for big and small objects
surprisingly for-of (H) solution is fast on chrome for small and big objects
solutions based on explicit index i (J,K) are quite fast on all browsers for small objects (for firefox also fast for big ojbects but medium fast on other browsers)
solutions based on iterators (D,E) are slowest and not recommended
solution C is slow for big objects and medium-slow for small objects
Details
Performance tests was performed for
small object - with 3 fields - you can perform test on your machine HERE
'big' object - with 1000 fields - you can perform test on your machine HERE
Below snippets presents used solutions
function A(obj,s='') {
for (let key in obj) if (obj.hasOwnProperty(key)) s+=key+'->'+obj[key] + ' ';
return s;
}
function B(obj,s='') {
for (let key in obj) s+=key+'->'+obj[key] + ' ';
return s;
}
function C(obj,s='') {
const map = new Map(Object.entries(obj));
for (let [key,value] of map) s+=key+'->'+value + ' ';
return s;
}
function D(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {
for (const i of Object.keys(this)) yield [i, this[i]];
}
}
for (let [key,value] of o) s+=key+'->'+value + ' ';
return s;
}
function E(obj,s='') {
let o = {
...obj,
*[Symbol.iterator]() {yield *Object.keys(this)}
}
for (let key of o) s+=key+'->'+o[key] + ' ';
return s;
}
function F(obj,s='') {
for (let key of Object.keys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function G(obj,s='') {
for (let [key, value] of Object.entries(obj)) s+=key+'->'+value+' ';
return s;
}
function H(obj,s='') {
for (let key of Object.getOwnPropertyNames(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function I(obj,s='') {
for (const key of Reflect.ownKeys(obj)) s+=key+'->'+obj[key]+' ';
return s;
}
function J(obj,s='') {
let keys = Object.keys(obj);
for(let i = 0; i < keys.length; i++){
let key = keys[i];
s+=key+'->'+obj[key]+' ';
}
return s;
}
function K(obj,s='') {
var keys = Object.keys(obj), len = keys.length, i = 0;
while (i < len) {
let key = keys[i];
s+=key+'->'+obj[key]+' ';
i += 1;
}
return s;
}
function L(obj,s='') {
Object.keys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
function M(obj,s='') {
Object.entries(obj).forEach(([key, value]) => s+=key+'->'+value+' ');
return s;
}
function N(obj,s='') {
Object.getOwnPropertyNames(obj).forEach(key => s+=key+'->'+obj[key]+' ');
return s;
}
function O(obj,s='') {
Reflect.ownKeys(obj).forEach(key=> s+=key+'->'+obj[key]+' ' );
return s;
}
// TEST
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
let log = (name,f) => console.log(`${name} ${f(p)}`)
log('A',A);
log('B',B);
log('C',C);
log('D',D);
log('E',E);
log('F',F);
log('G',G);
log('H',H);
log('I',I);
log('J',J);
log('K',K);
log('L',L);
log('M',M);
log('N',N);
log('O',O);
This snippet only presents choosen solutions
And here are result for small objects on chrome

for(key in p) {
alert( p[key] );
}
Note: you can do this over arrays, but you'll iterate over the length and other properties, too.

Since es2015 is getting more and more popular I am posting this answer which include usage of generator and iterator to smoothly iterate through [key, value] pairs. As it is possible in other languages for instance Ruby.
Ok here is a code:
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
[Symbol.iterator]: function*() {
for (const i of Object.keys(this)) {
yield [i, this[i]];
}
}
};
for (const [k, v] of MyObject) {
console.log(`Here is key ${k} and here is value ${v}`);
}
All information about how can you do an iterator and generator you can find at developer Mozilla page.
Hope It helped someone.
EDIT:
ES2017 will include Object.entries which will make iterating over [key, value] pairs in objects even more easier. It is now known that it will be a part of a standard according to the ts39 stage information.
I think it is time to update my answer to let it became even more fresher than it's now.
const MyObject = {
'a': 'Hello',
'b': 'it\'s',
'c': 'me',
'd': 'you',
'e': 'looking',
'f': 'for',
};
for (const [k, v] of Object.entries(MyObject)) {
console.log(`Here is key ${k} and here is value ${v}`);
}
You can find more about usage on
MDN page

Single line and more readable code can be..
Object.entries(myObject).map(([key, value]) => console.log(key, value))

After looking through all the answers in here, hasOwnProperty isn't required for my own usage because my json object is clean; there's really no sense in adding any additional javascript processing. This is all I'm using:
for (var key in p) {
console.log(key + ' => ' + p[key]);
// key is key
// value is p[key]
}

via prototype with forEach() which should skip the prototype chain properties:
Object.prototype.each = function(f) {
var obj = this
Object.keys(obj).forEach( function(key) {
f( key , obj[key] )
});
}
//print all keys and values
var obj = {a:1,b:2,c:3}
obj.each(function(key,value) { console.log(key + " " + value) });
// a 1
// b 2
// c 3

It's interesting people in these answers have touched on both Object.keys() and for...of but never combined them:
var map = {well:'hello', there:'!'};
for (let key of Object.keys(map))
console.log(key + ':' + map[key]);
You can't just for...of an Object because it's not an iterator, and for...index or .forEach()ing the Object.keys() is ugly/inefficient.
I'm glad most people are refraining from for...in (with or without checking .hasOwnProperty()) as that's also a bit messy, so other than my answer above, I'm here to say...
You can make ordinary object associations iterate! Behaving just like Maps with direct use of the fancy for...of
DEMO working in Chrome and FF (I assume ES6 only)
var ordinaryObject = {well:'hello', there:'!'};
for (let pair of ordinaryObject)
//key:value
console.log(pair[0] + ':' + pair[1]);
//or
for (let [key, value] of ordinaryObject)
console.log(key + ':' + value);
So long as you include my shim below:
//makes all objects iterable just like Maps!!! YAY
//iterates over Object.keys() (which already ignores prototype chain for us)
Object.prototype[Symbol.iterator] = function() {
var keys = Object.keys(this)[Symbol.iterator]();
var obj = this;
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
Without having to create a real Map object that doesn't have the nice syntactic sugar.
var trueMap = new Map([['well', 'hello'], ['there', '!']]);
for (let pair of trueMap)
console.log(pair[0] + ':' + pair[1]);
In fact, with this shim, if you still wanted to take advantage of Map's other functionality (without shimming them all in) but still wanted to use the neat object notation, since objects are now iterable you can now just make a Map from it!
//shown in demo
var realMap = new Map({well:'hello', there:'!'});
For those who don't like to shim, or mess with prototype in general, feel free to make the function on window instead, calling it something like getObjIterator() then;
//no prototype manipulation
function getObjIterator(obj) {
//create a dummy object instead of adding functionality to all objects
var iterator = new Object();
//give it what the shim does but as its own local property
iterator[Symbol.iterator] = function() {
var keys = Object.keys(obj)[Symbol.iterator]();
var output;
return {next:function() {
if (!(output = keys.next()).done)
output.value = [output.value, obj[output.value]];
return output;
}};
};
return iterator;
}
Now you can just call it as an ordinary function, nothing else is affected
var realMap = new Map(getObjIterator({well:'hello', there:'!'}))
or
for (let pair of getObjIterator(ordinaryObject))
There's no reason why that wouldn't work.
Welcome to the future.

Using a for-of on Object.keys()
Like:
let object = {
"key1": "value1",
"key2": "value2",
"key3": "value3"
};
for (let key of Object.keys(object)) {
console.log(key + " : " + object[key])
}

You can also use Object.keys() and iterate over the object keys like below to get the value:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach((key)=> {
console.log(key +' -> '+ p[key]);
});

var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (var key in p) {
if (p.hasOwnProperty(key)) {
console.log(key + " = " + p[key]);
}
}
<p>
Output:<br>
p1 = values1<br>
p2 = values2<br>
p3 = values3
</p>

Object.keys(obj) : Array
retrieves all string-valued keys of all enumerable own (non-inherited) properties.
So it gives the same list of keys as you intend by testing each object key with hasOwnProperty. You don't need that extra test operation than and Object.keys( obj ).forEach(function( key ){}) is supposed to be faster. Let's prove it:
var uniqid = function(){
var text = "",
i = 0,
possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
for( ; i < 32; i++ ) {
text += possible.charAt( Math.floor( Math.random() * possible.length ) );
}
return text;
},
CYCLES = 100000,
obj = {},
p1,
p2,
p3,
key;
// Populate object with random properties
Array.apply( null, Array( CYCLES ) ).forEach(function(){
obj[ uniqid() ] = new Date()
});
// Approach #1
p1 = performance.now();
Object.keys( obj ).forEach(function( key ){
var waste = obj[ key ];
});
p2 = performance.now();
console.log( "Object.keys approach took " + (p2 - p1) + " milliseconds.");
// Approach #2
for( key in obj ) {
if ( obj.hasOwnProperty( key ) ) {
var waste = obj[ key ];
}
}
p3 = performance.now();
console.log( "for...in/hasOwnProperty approach took " + (p3 - p2) + " milliseconds.");
In my Firefox I have following results
Object.keys approach took 40.21101451665163 milliseconds.
for...in/hasOwnProperty approach took 98.26163508463651 milliseconds.
PS. on Chrome the difference even bigger http://codepen.io/dsheiko/pen/JdrqXa
PS2: In ES6 (EcmaScript 2015) you can iterate iterable object nicer:
let map = new Map().set('a', 1).set('b', 2);
for (let pair of map) {
console.log(pair);
}
// OR
let map = new Map([
[false, 'no'],
[true, 'yes'],
]);
map.forEach((value, key) => {
console.log(key, value);
});

In latest ES script, you can do something like this:
let p = {foo: "bar"};
for (let [key, value] of Object.entries(p)) {
console.log(key, value);
}

Only JavaScript code without dependencies:
var p = {"p1": "value1", "p2": "value2", "p3": "value3"};
keys = Object.keys(p); // ["p1", "p2", "p3"]
for(i = 0; i < keys.length; i++){
console.log(keys[i] + "=" + p[keys[i]]); // p1=value1, p2=value2, p3=value3
}

The Object.keys() method returns an array of a given object's own enumerable properties. Read more about it here
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).map((key)=> console.log(key + "->" + p[key]))

Here is another method to iterate through an object.
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key => { console.log(key, p[key]) })

Multiple way to iterate object in javascript
Using for...in loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key in p){
if(p.hasOwnProperty(key)){
console.log(`${key} : ${p[key]}`)
}
}
Using for...of loop
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
for (let key of Object.keys(p)){
console.log(`key: ${key} & value: ${p[key]}`)
}
Using forEach() with Object.keys, Object.values, Object.entries
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3"
};
Object.keys(p).forEach(key=>{
console.log(`${key} : ${p[key]}`);
});
Object.values(p).forEach(value=>{
console.log(value);
});
Object.entries(p).forEach(([key,value])=>{
console.log(`${key}:${value}`)
})

Pass your object to Object.keys(). This will return an array containing all the keys in the object. You can then loop through the array using map. Using obj[key] where obj is your object and key is the current value in the map iteration, you can get the value for that key/property.
const obj = { name: "Jane", age: 50 };
Object.keys(obj).map( key => {
console.log(key, obj[key]);
});

Loops can be pretty interesting when using pure JavaScript. It seems that only ECMA6 (New 2015 JavaScript specification) got the loops under control. Unfortunately as I'm writing this, both Browsers and popular Integrated development environment (IDE) are still struggling to support completely the new bells and whistles.
At a glance here is what a JavaScript object loop look like before ECMA6:
for (var key in object) {
if (p.hasOwnProperty(key)) {
var value = object[key];
console.log(key); // This is the key;
console.log(value); // This is the value;
}
}
Also, I know this is out of scope with this question but in 2011, ECMAScript 5.1 added the forEach method for Arrays only which basically created a new improved way to loop through arrays while still leaving non iterable objects with the old verbose and confusing for loop. But the odd part is that this new forEach method does not support break which led to all sorts of other problems.
Basically in 2011, there is not a real solid way to loop in JavaScript other than what many popular libraries (jQuery, Underscore, etc.) decided to re-implement.
As of 2015, we now have a better out of the box way to loop (and break) any object type (including Arrays and Strings). Here is what a loop in JavaScript will eventually look like when the recommendation becomes mainstream:
for (let [key, value] of Object.entries(object)) {
console.log(key); // This is the key;
console.log(value); // This is the value;
}
Note that most browsers won't support the code above as of June 18th 2016. Even in Chrome you need to enable this special flag for it to work: chrome://flags/#enable-javascript-harmony
Until this becomes the new standard, the old method can still be used but there are also alternatives in popular libraries or even lightweight alternatives for those who aren't using any of these libraries.

You can add a simple forEach function to all objects, so you can automatically loop through any object:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
for (var key in this) {
if (!this.hasOwnProperty(key)) {
// skip loop if the property is from prototype
continue;
}
var value = this[key];
func(key, value);
}
},
enumerable: false
});
For those people who don't like the "for ... in"-method:
Object.defineProperty(Object.prototype, 'forEach', {
value: function (func) {
var arr = Object.keys(this);
for (var i = 0; i < arr.length; i++) {
var key = arr[i];
func(key, this[key]);
}
},
enumerable: false
});
Now, you can simple call:
p.forEach (function(key, value){
console.log ("Key: " + key);
console.log ("Value: " + value);
});
If you don't want to get conflicts with other forEach-Methods you can name it with your unique name.

Object.entries(myObject).map(([key, value]) => console.log(key, value))
You can try like this. myObject will be {name: "", phone: ""} so and so, this will generate key and value. So key here is name, phone and value are like dog, 123123.
Example {name: "dog"}
Here key is name and value is dog.

A good way for looping on an enumerable JavaScript object which could be awesome and common for ReactJS is using Object.keys or Object.entries with using map function. like below:
// assume items:
const items = {
first: { name: 'phone', price: 400 },
second: { name: 'tv', price: 300 },
third: { name: 'sofa', price: 250 },
};
For looping and show some UI on ReactJS act like below:
~~~
<div>
{Object.entries(items).map(([key, ({ name, price })]) => (
<div key={key}>
<span>name: {name}</span>
<span>price: {price}</span>
</div>
))}
</div>
Actually, I use the destructuring assignment twice, once for getting key once for getting name and price.

In ES6 we have well-known symbols to expose some previously internal methods, you can use it to define how iterators work for this object:
var p = {
"p1": "value1",
"p2": "value2",
"p3": "value3",
*[Symbol.iterator]() {
yield *Object.keys(this);
}
};
[...p] //["p1", "p2", "p3"]
this will give the same result as using for...in es6 loop.
for(var key in p) {
console.log(key);
}
But its important to know the capabilities you now have using es6!

I would do this rather than checking obj.hasOwnerProperty within every for ... in loop.
var obj = {a : 1};
for(var key in obj){
//obj.hasOwnProperty(key) is not needed.
console.log(key);
}
//then check if anybody has messed the native object. Put this code at the end of the page.
for(var key in Object){
throw new Error("Please don't extend the native object");
}