I have a JavaScript object. Is there a built-in or accepted best practice way to get the length of this object?
const myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
Updated answer
Here's an update as of 2016 and widespread deployment of ES5 and beyond. For IE9+ and all other modern ES5+ capable browsers, you can use Object.keys() so the above code just becomes:
var size = Object.keys(myObj).length;
This doesn't have to modify any existing prototype since Object.keys() is now built-in.
Edit: Objects can have symbolic properties that can not be returned via Object.key method. So the answer would be incomplete without mentioning them.
Symbol type was added to the language to create unique identifiers for object properties. The main benefit of the Symbol type is the prevention of overwrites.
Object.keys or Object.getOwnPropertyNames does not work for symbolic properties. To return them you need to use Object.getOwnPropertySymbols.
var person = {
[Symbol('name')]: 'John Doe',
[Symbol('age')]: 33,
"occupation": "Programmer"
};
const propOwn = Object.getOwnPropertyNames(person);
console.log(propOwn.length); // 1
let propSymb = Object.getOwnPropertySymbols(person);
console.log(propSymb.length); // 2
Older answer
The most robust answer (i.e. that captures the intent of what you're trying to do while causing the fewest bugs) would be:
Object.size = function(obj) {
var size = 0,
key;
for (key in obj) {
if (obj.hasOwnProperty(key)) size++;
}
return size;
};
// Get the size of an object
const myObj = {}
var size = Object.size(myObj);
There's a sort of convention in JavaScript that you don't add things to Object.prototype, because it can break enumerations in various libraries. Adding methods to Object is usually safe, though.
If you know you don't have to worry about hasOwnProperty checks, you can use the Object.keys() method in this way:
Object.keys(myArray).length
Updated: If you're using Underscore.js (recommended, it's lightweight!), then you can just do
_.size({one : 1, two : 2, three : 3});
=> 3
If not, and you don't want to mess around with Object properties for whatever reason, and are already using jQuery, a plugin is equally accessible:
$.assocArraySize = function(obj) {
// http://stackoverflow.com/a/6700/11236
var size = 0, key;
for (key in obj) {
if (obj.hasOwnProperty(key)) size++;
}
return size;
};
Here's the most cross-browser solution.
This is better than the accepted answer because it uses native Object.keys if exists.
Thus, it is the fastest for all modern browsers.
if (!Object.keys) {
Object.keys = function (obj) {
var arr = [],
key;
for (key in obj) {
if (obj.hasOwnProperty(key)) {
arr.push(key);
}
}
return arr;
};
}
Object.keys(obj).length;
Simply use this to get the length:
Object.keys(myObject).length
I'm not a JavaScript expert, but it looks like you would have to loop through the elements and count them since Object doesn't have a length method:
var element_count = 0;
for (e in myArray) { if (myArray.hasOwnProperty(e)) element_count++; }
#palmsey: In fairness to the OP, the JavaScript documentation actually explicitly refer to using variables of type Object in this manner as "associative arrays".
This method gets all your object's property names in an array, so you can get the length of that array which is equal to your object's keys' length.
Object.getOwnPropertyNames({"hi":"Hi","msg":"Message"}).length; // => 2
To not mess with the prototype or other code, you could build and extend your own object:
function Hash(){
var length=0;
this.add = function(key, val){
if(this[key] == undefined)
{
length++;
}
this[key]=val;
};
this.length = function(){
return length;
};
}
myArray = new Hash();
myArray.add("lastname", "Simpson");
myArray.add("age", 21);
alert(myArray.length()); // will alert 2
If you always use the add method, the length property will be correct. If you're worried that you or others forget about using it, you could add the property counter which the others have posted to the length method, too.
Of course, you could always overwrite the methods. But even if you do, your code would probably fail noticeably, making it easy to debug. ;)
We can find the length of Object by using:
const myObject = {};
console.log(Object.values(myObject).length);
Here's how and don't forget to check that the property is not on the prototype chain:
var element_count = 0;
for(var e in myArray)
if(myArray.hasOwnProperty(e))
element_count++;
Here is a completely different solution that will only work in more modern browsers (Internet Explorer 9+, Chrome, Firefox 4+, Opera 11.60+, and Safari 5.1+)
See this jsFiddle.
Setup your associative array class
/**
* #constructor
*/
AssociativeArray = function () {};
// Make the length property work
Object.defineProperty(AssociativeArray.prototype, "length", {
get: function () {
var count = 0;
for (var key in this) {
if (this.hasOwnProperty(key))
count++;
}
return count;
}
});
Now you can use this code as follows...
var a1 = new AssociativeArray();
a1["prop1"] = "test";
a1["prop2"] = 1234;
a1["prop3"] = "something else";
alert("Length of array is " + a1.length);
If you need an associative data structure that exposes its size, better use a map instead of an object.
const myMap = new Map();
myMap.set("firstname", "Gareth");
myMap.set("lastname", "Simpson");
myMap.set("age", 21);
console.log(myMap.size); // 3
Use Object.keys(myObject).length to get the length of object/array
var myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
console.log(Object.keys(myObject).length); //3
Use:
var myArray = new Object();
myArray["firstname"] = "Gareth";
myArray["lastname"] = "Simpson";
myArray["age"] = 21;
obj = Object.keys(myArray).length;
console.log(obj)
<script>
myObj = {"key1" : "Hello", "key2" : "Goodbye"};
var size = Object.keys(myObj).length;
console.log(size);
</script>
<p id="myObj">The number of <b>keys</b> in <b>myObj</b> are: <script>document.write(size)</script></p>
This works for me:
var size = Object.keys(myObj).length;
For some cases it is better to just store the size in a separate variable. Especially, if you're adding to the array by one element in one place and can easily increment the size. It would obviously work much faster if you need to check the size often.
The simplest way is like this:
Object.keys(myobject).length
Where myobject is the object of what you want the length of.
#palmsey: In fairness to the OP, the JavaScript documentation actually explicitly refer to using variables of type Object in this manner as "associative arrays".
And in fairness to #palmsey he was quite correct. They aren't associative arrays; they're definitely objects :) - doing the job of an associative array. But as regards to the wider point, you definitely seem to have the right of it according to this rather fine article I found:
JavaScript “Associative Arrays” Considered Harmful
But according to all this, the accepted answer itself is bad practice?
Specify a prototype size() function for Object
If anything else has been added to Object .prototype, then the suggested code will fail:
<script type="text/javascript">
Object.prototype.size = function () {
var len = this.length ? --this.length : -1;
for (var k in this)
len++;
return len;
}
Object.prototype.size2 = function () {
var len = this.length ? --this.length : -1;
for (var k in this)
len++;
return len;
}
var myArray = new Object();
myArray["firstname"] = "Gareth";
myArray["lastname"] = "Simpson";
myArray["age"] = 21;
alert("age is " + myArray["age"]);
alert("length is " + myArray.size());
</script>
I don't think that answer should be the accepted one as it can't be trusted to work if you have any other code running in the same execution context. To do it in a robust fashion, surely you would need to define the size method within myArray and check for the type of the members as you iterate through them.
If we have the hash
hash = {"a" : "b", "c": "d"};
we can get the length using the length of the keys which is the length of the hash:
keys(hash).length
Using the Object.entries method to get length is one way of achieving it
const objectLength = obj => Object.entries(obj).length;
const person = {
id: 1,
name: 'John',
age: 30
}
const car = {
type: 2,
color: 'red',
}
console.log(objectLength(person)); // 3
console.log(objectLength(car)); // 2
var myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
Object.values(myObject).length
Object.entries(myObject).length
Object.keys(myObject).length
What about something like this --
function keyValuePairs() {
this.length = 0;
function add(key, value) { this[key] = value; this.length++; }
function remove(key) { if (this.hasOwnProperty(key)) { delete this[key]; this.length--; }}
}
If you are using AngularJS 1.x you can do things the AngularJS way by creating a filter and using the code from any of the other examples such as the following:
// Count the elements in an object
app.filter('lengthOfObject', function() {
return function( obj ) {
var size = 0, key;
for (key in obj) {
if (obj.hasOwnProperty(key)) size++;
}
return size;
}
})
Usage
In your controller:
$scope.filterResult = $filter('lengthOfObject')($scope.object)
Or in your view:
<any ng-expression="object | lengthOfObject"></any>
const myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
console.log(Object.keys(myObject).length)
// o/p 3
A variation on some of the above is:
var objLength = function(obj){
var key,len=0;
for(key in obj){
len += Number( obj.hasOwnProperty(key) );
}
return len;
};
It is a bit more elegant way to integrate hasOwnProp.
If you don't care about supporting Internet Explorer 8 or lower, you can easily get the number of properties in an object by applying the following two steps:
Run either Object.keys() to get an array that contains the names of only those properties that are enumerable or Object.getOwnPropertyNames() if you want to also include the names of properties that are not enumerable.
Get the .length property of that array.
If you need to do this more than once, you could wrap this logic in a function:
function size(obj, enumerablesOnly) {
return enumerablesOnly === false ?
Object.getOwnPropertyNames(obj).length :
Object.keys(obj).length;
}
How to use this particular function:
var myObj = Object.create({}, {
getFoo: {},
setFoo: {}
});
myObj.Foo = 12;
var myArr = [1,2,5,4,8,15];
console.log(size(myObj)); // Output : 1
console.log(size(myObj, true)); // Output : 1
console.log(size(myObj, false)); // Output : 3
console.log(size(myArr)); // Output : 6
console.log(size(myArr, true)); // Output : 6
console.log(size(myArr, false)); // Output : 7
See also this Fiddle for a demo.
Here's a different version of James Cogan's answer. Instead of passing an argument, just prototype out the Object class and make the code cleaner.
Object.prototype.size = function () {
var size = 0,
key;
for (key in this) {
if (this.hasOwnProperty(key)) size++;
}
return size;
};
var x = {
one: 1,
two: 2,
three: 3
};
x.size() === 3;
jsfiddle example: http://jsfiddle.net/qar4j/1/
You can always do Object.getOwnPropertyNames(myObject).length to get the same result as [].length would give for normal array.
You can simply use Object.keys(obj).length on any object to get its length. Object.keys returns an array containing all of the object keys (properties) which can come in handy for finding the length of that object using the length of the corresponding array. You can even write a function for this. Let's get creative and write a method for it as well (along with a more convienient getter property):
function objLength(obj)
{
return Object.keys(obj).length;
}
console.log(objLength({a:1, b:"summit", c:"nonsense"}));
// Works perfectly fine
var obj = new Object();
obj['fish'] = 30;
obj['nullified content'] = null;
console.log(objLength(obj));
// It also works your way, which is creating it using the Object constructor
Object.prototype.getLength = function() {
return Object.keys(this).length;
}
console.log(obj.getLength());
// You can also write it as a method, which is more efficient as done so above
Object.defineProperty(Object.prototype, "length", {get:function(){
return Object.keys(this).length;
}});
console.log(obj.length);
// probably the most effictive approach is done so and demonstrated above which sets a getter property called "length" for objects which returns the equivalent value of getLength(this) or this.getLength()
A nice way to achieve this (Internet Explorer 9+ only) is to define a magic getter on the length property:
Object.defineProperty(Object.prototype, "length", {
get: function () {
return Object.keys(this).length;
}
});
And you can just use it like so:
var myObj = { 'key': 'value' };
myObj.length;
It would give 1.
I know all about JSON.stringify or JSON.parse in the sense that one serializes an object and one deserializes the string back into an object. This is great!
However, I have the following situation:
var i = new MyMagicalObject();
var oi = JSON.parse(JSON.stringify(i));
console.log(i.numFields()); // this is fine
console.log(oi.numFields()); // this throws since Object has no method 'numFields'
Basically, I'd like to treat oi as an instance of "MyMagicalObject" since that's what it is.
I'm sure there's some magic about setting the prototype on oi or something, but I'm fairly new to JavaScript. Any help would be appreciated.
You can't "store" JavaScript functions in JSON strings.
The only data types that can be stored in JSON are:
Number
String
Boolean
Array
Object
null
(source)
Anything that isn't one of those types, gets ignored:
function Test(){
this.foo = function(){
return 'bar';
}
this.theAnswer = '42';
}
var t = new Test();
alert(t.foo());
alert(JSON.stringify(t))
Your problem could be easily solved by redesigning your MyMagicalObject class. Here is an example of JSON-friendly class:
function MyMagicalObject(props) {
this.props = props || {};
}
MyMagicalObject.prototype.get = function(key) {
return this.props[key];
};
MyMagicalObject.prototype.set = function(key, val) {
this.props[key] = val;
return this;
};
MyMagicalObject.prototype.toJSON = function() {
return this.props;
};
MyMagicalObject.prototype.numFields = function() {
return Object.keys(this.props).length;
};
This realization follows two rules:
It's constructor accepts JSON representation as a first argument.
It provides toJSON method to tell JS engine how to convert its instance to JSON.
Check the following example:
var obj = new MyMagicalObject();
obj.set('foo', 42).set('bar', 'baz');
alert(obj.numFields()); // 2
var str = JSON.stringify(obj);
var obj2 = new MyMagicalObject(JSON.parse(str));
alert(obj2.numFields()); // 2
You can create a new MyMagicalObject() and then overwrite its properties with the one from oi.
var t = new MyMagicalObject();
for(var k in oi) t[k]=oi[k];
That should do the trick. If you have a more complex object (with more than 1 dimension), search for a copy function that deep copies all properties.
Add oi.prototype = MyMagicalObject.prototype; after line 3.
or
create a new object and copy the properties:
var oi2 = new MyMagicalObject();
for (var p in oi) {
if (oi.hasOwnProperty(p)) {
oi2[p] = oi[p]
}
}
console.log(oi2.numFields());
This question already has answers here:
How to use a variable for a key in a JavaScript object literal?
(16 answers)
Closed 8 years ago.
First off, I'm using Cheerio for some DOM access and parsing with Node.js. Good times.
Heres the situation:
I have a function that I need to create an object. That object uses variables for both its keys and values, and then return that single object. Example:
stuff = function (thing, callback) {
var inputs = $('div.quantity > input').map(function(){
var key = this.attr('name')
, value = this.attr('value');
return { key : value }
})
callback(null, inputs);
}
It outputs this:
[ { key: '1' }, { key: '1' } ]
(.map() returns an array of objects fyi)
I need key to actually be the string from this.attr('name').
Whats the best way to assign a string as a key in Javascript, considering what I'm trying to do?
In the new ES2015 standard for JavaScript (formerly called ES6), objects can be created with computed keys: Object Initializer spec.
The syntax is:
var obj = {
[myKey]: value,
}
If applied to the OP's scenario, it would turn into:
stuff = function (thing, callback) {
var inputs = $('div.quantity > input').map(function(){
return {
[this.attr('name')]: this.attr('value'),
};
})
callback(null, inputs);
}
Note: A transpiler is still required for browser compatiblity.
Using Babel or Google's traceur, it is possible to use this syntax today.
In earlier JavaScript specifications (ES5 and below), the key in an object literal is always interpreted literally, as a string.
To use a "dynamic" key, you have to use bracket notation:
var obj = {};
obj[myKey] = value;
In your case:
stuff = function (thing, callback) {
var inputs = $('div.quantity > input').map(function(){
var key = this.attr('name')
, value = this.attr('value')
, ret = {};
ret[key] = value;
return ret;
})
callback(null, inputs);
}
You can't define an object literal with a dynamic key. Do this :
var o = {};
o[key] = value;
return o;
There's no shortcut (edit: there's one now, with ES6, see the other answer).
I am doing a project which requires to pass Perl objects to javascript via JSON. I am facing a problem in terms of "intermediate" object definition.
In Perl, object is represented by hash, and programmers don't have to define anything "in the middle". Once a property is created, all intermediate objects are automatically created as hash references. e.g.
$graph{chart}{yAxis}{title} = "Temperature Tracking";
However, once this object is passed to Javascript, if I want to add any new properties in the "intermediate" object, like:
graph.chart.xAxis.title = "Time Sequence";
I'll have an "undefined graph.chart.xAxis" error. Unlike Perl, Javascript doesn't automatically create objects if we simply assign a property for it.
At the moment I have to use below solution:
if (!graph.chart.xAxis) {
graph.chart.xAxis = {};
graph.chart.xAxis.title = "Time Sequence";
}
Unfortunately, in our project the objects passed from Perl are pretty dynamic and there are plenty of other objects that Javascript may not know. Above way makes JS code pretty lengthy and "ugly looking". Are there any better solutions to make Javascript behave like Perl, which means I don't have to create intermediate objects manually?
I'm not sure whether this meets your requirements but a simple function to create the missing objects could look like this:
function insertNode(obj, node, value) {
var segments = node.split('.');
var key = segments.pop();
var ref = obj;
while (segments.length > 0) {
var segment = segments.shift();
if (typeof ref[segment] != 'object') {
ref[segment] = {};
}
ref = ref[segment];
}
ref[key] = value;
};
var x = {};
insertNode(x, 'foo.bar.baz', 'hello world');
alert(x.foo.bar.baz); // "hello world"
Demo: http://jsfiddle.net/sNywt/1/
You may be interested in a library called steeltoe
steelToe(graph).set('chart.xAxis.title', 'Time Sequence');
not sure, if its suitable in your case, but you could check for property existence and create it if does not exist, like:
function use_or_create(obj, prop) {
return (obj.hasOwnProperty(prop)) ? true : (obj[prop] = {});
}
var graph.chart = {}; //your object
//function call to check if property exist before trying to use it
use_or_create(graph.chart, 'xAxis'); //check if xAxis exists and creates one if doesnot
graph.char.xAxis.title = "tested";
Maybe this Object extension will do:
Object.prototype.val = function(prop,val){
prop = /\./i.test(prop) ? prop.split('.') : prop;
if (prop.constructor === Array){
var objnow = this, pr;
while (pr = prop.shift()){
if (!objnow[pr]){
objnow[pr] = {};
}
if (!prop.length) {
objnow[pr] = val;
}
objnow = objnow[pr];
}
for (var l in objnow){
this[l] = objnow[l];
}
} else {
this[prop] = val;
}
}
// usage
var myO = {};
myO.val('a.b.c',3); //=> myO.a.b.c = 3
myO.val('someprop',3); //=> myO.someprop = 3
myO.val('a.b.someprop',5); //=> myO.a.b.someprop = 3
I am trying to remove an element from a Javascript associtive array using the value to find it, but I am having trouble. I have tried splice and JQuery's grep method and neither have worked for me. This is what I currently have.
var array_path = new Array();
function bulk_upload(){
var temp_array = new Object();
for (var i = 1; i<8; i++){
temp_array[i] = $('#path' + i).val();
if(temp_array[i]!='' && temp_array[i]!=null){
array_path['path' + i] = $('#path' + i).val();
}
}
process_txt();
}
function process_txt(){
//alert(array_path.indexOf(full_path)); //returns nothing
var removed_element = array_path.splice(getKey(array_path), 1);
//array_path = $.grep(array_path, function(val) { return val != full_path; });
alert(removed_element);//return nothing, just blank alert box
}
function getKey(data) {
for (var prop in data)
return prop;
}
The way to do this is to use the delete operator.
delete array_path[getKey(array_path)]
Some Background Information
In JavaScript, almost everything descends from Object.prototype. JavaScript, being an open and dynamic language allows you to create/modify properties of objects by simple assignment. This is very similar to what an associative array -- a structure that contains keyed values.
Under the hood an array is just an object that descends from Array.prototype with numeric keys and a special property called length. The length property just returns one greater than the highest numeric property. In essence, an Array is an object with different semantics.
If you're wanting an associative array then Array is not the object you want to descend from. You would want to descend directly from Object. There are two ways to do that, you could either use the new operator or an empty object literal. The syntax for both is below:
var o = new Object();
var o = {};
The second is preferred since it's a little bit more concise.
I wrote a blog post about this a while back, have a look if you want a little bit more info.
There is no such thing in JavaScript as an "associative array" per se. The data structure which corresponds to this concept is simply a JavaScript Object.
Of course, a JavaScript Array (like essentially everything in JavaScript) is an Object, but one with additional capabilities. So you can use an Array as a key-value map, but it's really not the correct structure for that.
To remove a key from an Object, you just do something like this:
var myObj = {};
var myKey = "blah";
myObj[myKey] = 1234; // Adds or updates value for "blah" to 1234.
delete myObj[myKey]; // Removes key-value pair for "blah".
Have you tried delete hash.someKey; ?
You can give your object a remove method, or use apply or call to use another object's remove method, if defined.
function myObj(members){
for(var p in members) this[p]= members[p];
}
myObj.prototype.remove= function(val){
for(var p in this){
if(this[p]=== val) delete this[p];
}
return this;
}
myObj.prototype.toString= function(){
var A= [];;
for(var p in this){
if(this.hasOwnProperty(p)){
A.push(p+':'+this[p])
}
}
return '{'+A.join(', ')+'}';
}
var O= new myObj({a: 1, b: 10, c: 100});
alert(O)
O.remove(10);
alert(O)
I'm not psychic, so I can only guess that you wanted to accomplish something like this:
var paths = [];
function getPaths() {
for(var i = 1; i < 8; ++i) {
var value = $('#path' + i).val();
if(value) paths.push(value);
}
}
function process() {
var firstPath = paths.shift();
// do stuff
}
getPaths();
if(paths.length) process();