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How to return an Object Value if you have a specific key in your Object? [duplicate]

How do I check if an object has a specific property in JavaScript?
Consider:
x = {'key': 1};
if ( x.hasOwnProperty('key') ) {
//Do this
}
Is that the best way to do it?

2022 UPDATE
Object.hasOwn()
Object.hasOwn() is recommended over Object.hasOwnProperty() because it works for objects created using Object.create(null) and with objects that have overridden the inherited hasOwnProperty() method. While it is possible to workaround these problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() is more intuitive.
Example
const object1 = {
prop: 'exists'
};
console.log(Object.hasOwn(object1, 'prop'));
// expected output: true
Original answer
I'm really confused by the answers that have been given - most of them are just outright incorrect. Of course you can have object properties that have undefined, null, or false values. So simply reducing the property check to typeof this[property] or, even worse, x.key will give you completely misleading results.
It depends on what you're looking for. If you want to know if an object physically contains a property (and it is not coming from somewhere up on the prototype chain) then object.hasOwnProperty is the way to go. All modern browsers support it. (It was missing in older versions of Safari - 2.0.1 and older - but those versions of the browser are rarely used any more.)
If what you're looking for is if an object has a property on it that is iterable (when you iterate over the properties of the object, it will appear) then doing: prop in object will give you your desired effect.
Since using hasOwnProperty is probably what you want, and considering that you may want a fallback method, I present to you the following solution:
var obj = {
a: undefined,
b: null,
c: false
};
// a, b, c all found
for ( var prop in obj ) {
document.writeln( "Object1: " + prop );
}
function Class(){
this.a = undefined;
this.b = null;
this.c = false;
}
Class.prototype = {
a: undefined,
b: true,
c: true,
d: true,
e: true
};
var obj2 = new Class();
// a, b, c, d, e found
for ( var prop in obj2 ) {
document.writeln( "Object2: " + prop );
}
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
if ( Object.prototype.hasOwnProperty ) {
var hasOwnProperty = function(obj, prop) {
return obj.hasOwnProperty(prop);
}
}
// a, b, c found in modern browsers
// b, c found in Safari 2.0.1 and older
for ( var prop in obj2 ) {
if ( hasOwnProperty(obj2, prop) ) {
document.writeln( "Object2 w/ hasOwn: " + prop );
}
}
The above is a working, cross-browser, solution to hasOwnProperty(), with one caveat: It is unable to distinguish between cases where an identical property is on the prototype and on the instance - it just assumes that it's coming from the prototype. You could shift it to be more lenient or strict, based upon your situation, but at the very least this should be more helpful.

With Underscore.js or (even better) Lodash:
_.has(x, 'key');
Which calls Object.prototype.hasOwnProperty, but (a) is shorter to type, and (b) uses "a safe reference to hasOwnProperty" (i.e. it works even if hasOwnProperty is overwritten).
In particular, Lodash defines _.has as:
function has(object, key) {
return object ? hasOwnProperty.call(object, key) : false;
}
// hasOwnProperty = Object.prototype.hasOwnProperty

You can use this (but read the warning below):
var x = {
'key': 1
};
if ('key' in x) {
console.log('has');
}
But be warned: 'constructor' in x will return true even if x is an empty object - same for 'toString' in x, and many others. It's better to use Object.hasOwn(x, 'key').

Note: the following is nowadays largely obsolete thanks to strict mode, and hasOwnProperty. The correct solution is to use strict mode and to check for the presence of a property using obj.hasOwnProperty. This answer predates both these things, at least as widely implemented (yes, it is that old). Take the following as a historical note.
Bear in mind that undefined is (unfortunately) not a reserved word in JavaScript if you’re not using strict mode. Therefore, someone (someone else, obviously) could have the grand idea of redefining it, breaking your code.
A more robust method is therefore the following:
if (typeof(x.attribute) !== 'undefined')
On the flip side, this method is much more verbose and also slower. :-/
A common alternative is to ensure that undefined is actually undefined, e.g. by putting the code into a function which accepts an additional parameter, called undefined, that isn’t passed a value. To ensure that it’s not passed a value, you could just call it yourself immediately, e.g.:
(function (undefined) {
… your code …
if (x.attribute !== undefined)
… mode code …
})();

if (x.key !== undefined)
Armin Ronacher seems to have already beat me to it, but:
Object.prototype.hasOwnProperty = function(property) {
return this[property] !== undefined;
};
x = {'key': 1};
if (x.hasOwnProperty('key')) {
alert('have key!');
}
if (!x.hasOwnProperty('bar')) {
alert('no bar!');
}
A safer, but slower solution, as pointed out by Konrad Rudolph and Armin Ronacher would be:
Object.prototype.hasOwnProperty = function(property) {
return typeof this[property] !== 'undefined';
};

Considering the following object in Javascript
const x = {key: 1};
You can use the in operator to check if the property exists on an object:
console.log("key" in x);
You can also loop through all the properties of the object using a for - in loop, and then check for the specific property:
for (const prop in x) {
if (prop === "key") {
//Do something
}
}
You must consider if this object property is enumerable or not, because non-enumerable properties will not show up in a for-in loop. Also, if the enumerable property is shadowing a non-enumerable property of the prototype, it will not show up in Internet Explorer 8 and earlier.
If you’d like a list of all instance properties, whether enumerable or not, you can use
Object.getOwnPropertyNames(x);
This will return an array of names of all properties that exist on an object.
Reflections provide methods that can be used to interact with Javascript objects. The static Reflect.has() method works like the in operator as a function.
console.log(Reflect.has(x, 'key'));
// expected output: true
console.log(Reflect.has(x, 'key2'));
// expected output: false
console.log(Reflect.has(object1, 'toString'));
// expected output: true
Finally, you can use the typeof operator to directly check the data type of the object property:
if (typeof x.key === "undefined") {
console.log("undefined");
}
If the property does not exist on the object, it will return the string undefined. Else it will return the appropriate property type. However, note that this is not always a valid way of checking if an object has a property or not, because you could have a property that is set to undefined, in which case, using typeof x.key would still return true (even though the key is still in the object).
Similarly, you can check if a property exists by comparing directly to the undefined Javascript property
if (x.key === undefined) {
console.log("undefined");
}
This should work unless key was specifically set to undefined on the x object

Let's cut through some confusion here. First, let's simplify by assuming hasOwnProperty already exists; this is true of the vast majority of current browsers in use.
hasOwnProperty returns true if the attribute name that is passed to it has been added to the object. It is entirely independent of the actual value assigned to it which may be exactly undefined.
Hence:
var o = {}
o.x = undefined
var a = o.hasOwnProperty('x') // a is true
var b = o.x === undefined // b is also true
However:
var o = {}
var a = o.hasOwnProperty('x') // a is now false
var b = o.x === undefined // b is still true
The problem is what happens when an object in the prototype chain has an attribute with the value of undefined? hasOwnProperty will be false for it, and so will !== undefined. Yet, for..in will still list it in the enumeration.
The bottom line is there is no cross-browser way (since Internet Explorer doesn't expose __prototype__) to determine that a specific identifier has not been attached to an object or anything in its prototype chain.

If you are searching for a property, then "no". You want:
if ('prop' in obj) { }
In general, you should not care whether or not the property comes from the prototype or the object.
However, because you used 'key' in your sample code, it looks like you are treating the object as a hash, in which case your answer would make sense. All of the hashes keys would be properties in the object, and you avoid the extra properties contributed by the prototype.
John Resig's answer was very comprehensive, but I thought it wasn't clear. Especially with when to use "'prop' in obj".

For testing simple objects, use:
if (obj[x] !== undefined)
If you don't know what object type it is, use:
if (obj.hasOwnProperty(x))
All other options are slower...
Details
A performance evaluation of 100,000,000 cycles under Node.js to the five options suggested by others here:
function hasKey1(k,o) { return (x in obj); }
function hasKey2(k,o) { return (obj[x]); }
function hasKey3(k,o) { return (obj[x] !== undefined); }
function hasKey4(k,o) { return (typeof(obj[x]) !== 'undefined'); }
function hasKey5(k,o) { return (obj.hasOwnProperty(x)); }
The evaluation tells us that unless we specifically want to check the object's prototype chain as well as the object itself, we should not use the common form:
if (X in Obj)...
It is between 2 to 6 times slower depending on the use case
hasKey1 execution time: 4.51 s
hasKey2 execution time: 0.90 s
hasKey3 execution time: 0.76 s
hasKey4 execution time: 0.93 s
hasKey5 execution time: 2.15 s
Bottom line, if your Obj is not necessarily a simple object and you wish to avoid checking the object's prototype chain and to ensure x is owned by Obj directly, use if (obj.hasOwnProperty(x))....
Otherwise, when using a simple object and not being worried about the object's prototype chain, using if (typeof(obj[x]) !== 'undefined')... is the safest and fastest way.
If you use a simple object as a hash table and never do anything kinky, I would use if (obj[x])... as I find it much more readable.

Yes it is :) I think you can also do Object.prototype.hasOwnProperty.call(x, 'key') which should also work if x has a property called hasOwnProperty :)
But that tests for own properties. If you want to check if it has an property that may also be inhered you can use typeof x.foo != 'undefined'.

if(x.hasOwnProperty("key")){
// …
}
because
if(x.key){
// …
}
fails if x.key is falsy (for example, x.key === "").

You can also use the ES6 Reflect object:
x = {'key': 1};
Reflect.has( x, 'key'); // returns true
Documentation on MDN for Reflect.has can be found here.
The static Reflect.has() method works like the in operator as a function.

Do not do this object.hasOwnProperty(key)). It's really bad because these methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
The best way is to do Object.prototype.hasOwnProperty.call(object, key) or:
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(object, key));
/* Or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));

OK, it looks like I had the right answer unless if you don't want inherited properties:
if (x.hasOwnProperty('key'))
Here are some other options to include inherited properties:
if (x.key) // Quick and dirty, but it does the same thing as below.
if (x.key !== undefined)

Another relatively simple way is using Object.keys. This returns an array which means you get all of the features of an array.
var noInfo = {};
var info = {something: 'data'};
Object.keys(noInfo).length //returns 0 or false
Object.keys(info).length //returns 1 or true
Although we are in a world with great browser support. Because this question is so old I thought I'd add this:
This is safe to use as of JavaScript v1.8.5.

JavaScript is now evolving and growing as it now has good and even efficient ways to check it.
Here are some easy ways to check if object has a particular property:
Using hasOwnProperty()
const hero = {
name: 'Batman'
};
hero.hasOwnProperty('name'); // => true
hero.hasOwnProperty('realName'); // => false
Using keyword/operator in
const hero = {
name: 'Batman'
};
'name' in hero; // => true
'realName' in hero; // => false
Comparing with undefined keyword
const hero = {
name: 'Batman'
};
hero.name; // => 'Batman'
hero.realName; // => undefined
// So consider this
hero.realName == undefined // => true (which means property does not exists in object)
hero.name == undefined // => false (which means that property exists in object)
For more information, check here.

hasOwnProperty "can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check down the object's prototype chain."
So most probably, for what seems by your question, you don't want to use hasOwnProperty, which determines if the property exists as attached directly to the object itself,.
If you want to determine if the property exists in the prototype chain, you may want to use it like:
if (prop in object) { // Do something }

You can use the following approaches-
var obj = {a:1}
console.log('a' in obj) // 1
console.log(obj.hasOwnProperty('a')) // 2
console.log(Boolean(obj.a)) // 3
The difference between the following approaches are as follows-
In the first and third approach we are not just searching in object but its prototypal chain too. If the object does not have the property, but the property is present in its prototype chain it is going to give true.
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log('b' in obj)
console.log(Boolean(obj.b))
The second approach will check only for its own properties. Example -
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log(obj.hasOwnProperty('b'))
The difference between the first and the third is if there is a property which has value undefined the third approach is going to give false while first will give true.
var obj = {
b : undefined
}
console.log(Boolean(obj.b))
console.log('b' in obj);

Given myObject object and “myKey” as key name:
Object.keys(myObject).includes('myKey')
or
myObject.hasOwnProperty('myKey')
or
typeof myObject.myKey !== 'undefined'
The last was widely used, but (as pointed out in other answers and comments) it could also match on keys deriving from Object prototype.

Performance
Today 2020.12.17 I perform tests on MacOs HighSierra 10.13.6 on Chrome v87, Safari v13.1.2 and Firefox v83 for chosen solutions.
Results
I compare only solutions A-F because they give valid result for all cased used in snippet in details section. For all browsers
solution based on in (A) is fast or fastest
solution (E) is fastest for chrome for big objects and fastest for firefox for small arrays if key not exists
solution (F) is fastest (~ >10x than other solutions) for small arrays
solutions (D,E) are quite fast
solution based on losash has (B) is slowest
Details
I perform 4 tests cases:
when object has 10 fields and searched key exists - you can run it HERE
when object has 10 fields and searched key not exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
Below snippet presents differences between solutions
A
B
C
D
E
F
G
H
I
J
K
// SO https://stackoverflow.com/q/135448/860099
// src: https://stackoverflow.com/a/14664748/860099
function A(x) {
return 'key' in x
}
// src: https://stackoverflow.com/a/11315692/860099
function B(x) {
return _.has(x, 'key')
}
// src: https://stackoverflow.com/a/40266120/860099
function C(x) {
return Reflect.has( x, 'key')
}
// src: https://stackoverflow.com/q/135448/860099
function D(x) {
return x.hasOwnProperty('key')
}
// src: https://stackoverflow.com/a/11315692/860099
function E(x) {
return Object.prototype.hasOwnProperty.call(x, 'key')
}
// src: https://stackoverflow.com/a/136411/860099
function F(x) {
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
return hasOwnProperty(x,'key')
}
// src: https://stackoverflow.com/a/135568/860099
function G(x) {
return typeof(x.key) !== 'undefined'
}
// src: https://stackoverflow.com/a/22740939/860099
function H(x) {
return x.key !== undefined
}
// src: https://stackoverflow.com/a/38332171/860099
function I(x) {
return !!x.key
}
// src: https://stackoverflow.com/a/41184688/860099
function J(x) {
return !!x['key']
}
// src: https://stackoverflow.com/a/54196605/860099
function K(x) {
return Boolean(x.key)
}
// --------------------
// TEST
// --------------------
let x1 = {'key': 1};
let x2 = {'key': "1"};
let x3 = {'key': true};
let x4 = {'key': []};
let x5 = {'key': {}};
let x6 = {'key': ()=>{}};
let x7 = {'key': ''};
let x8 = {'key': 0};
let x9 = {'key': false};
let x10= {'key': undefined};
let x11= {'nokey': 1};
let b= x=> x ? 1:0;
console.log(' 1 2 3 4 5 6 7 8 9 10 11');
[A,B,C,D,E,F,G,H,I,J,K ].map(f=> {
console.log(
`${f.name} ${b(f(x1))} ${b(f(x2))} ${b(f(x3))} ${b(f(x4))} ${b(f(x5))} ${b(f(x6))} ${b(f(x7))} ${b(f(x8))} ${b(f(x9))} ${b(f(x10))} ${b(f(x11))} `
)})
console.log('\nLegend: Columns (cases)');
console.log('1. key = 1 ');
console.log('2. key = "1" ');
console.log('3. key = true ');
console.log('4. key = [] ');
console.log('5. key = {} ');
console.log('6. key = ()=>{} ');
console.log('7. key = "" ');
console.log('8. key = 0 ');
console.log('9. key = false ');
console.log('10. key = undefined ');
console.log('11. no-key ');
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"> </script>
This shippet only presents functions used in performance tests - it not perform tests itself!
And here are example results for chrome

Now with ECMAScript22 we can use hasOwn instead of hasOwnProperty (Because this feature has pitfalls )
Object.hasOwn(obj, propKey)

Here is another option for a specific case. :)
If you want to test for a member on an object and want to know if it has been set to something other than:
''
false
null
undefined
0
...
then you can use:
var foo = {};
foo.bar = "Yes, this is a proper value!";
if (!!foo.bar) {
// member is set, do something
}

some easier and short options depending on the specific use case:
to check if the property exists, regardless of value, use the in operator ("a" in b)
to check a property value from a variable, use bracket notation (obj[v])
to check a property value as truthy, use optional
chaining (?.)
to check a property value boolean, use double-not / bang-bang / (!!)
to set a default value for null / undefined check, use nullish coalescing operator (??)
to set a default value for falsey value check, use short-circuit logical OR operator (||)
run the code snippet to see results:
let obj1 = {prop:undefined};
console.log(1,"prop" in obj1);
console.log(1,obj1?.prop);
let obj2 = undefined;
//console.log(2,"prop" in obj2); would throw because obj2 undefined
console.log(2,"prop" in (obj2 ?? {}))
console.log(2,obj2?.prop);
let obj3 = {prop:false};
console.log(3,"prop" in obj3);
console.log(3,!!obj3?.prop);
let obj4 = {prop:null};
let look = "prop"
console.log(4,"prop" in obj4);
console.log(4,obj4?.[look]);
let obj5 = {prop:true};
console.log(5,"prop" in obj5);
console.log(5,obj5?.prop === true);
let obj6 = {otherProp:true};
look = "otherProp"
console.log(6,"prop" in obj6);
console.log(6,obj6.look); //should have used bracket notation
let obj7 = {prop:""};
console.log(7,"prop" in obj7);
console.log(7,obj7?.prop || "empty");
I see very few instances where hasOwn is used properly, especially given its inheritance issues

There is a method, "hasOwnProperty", that exists on an object, but it's not recommended to call this method directly, because it might be sometimes that the object is null or some property exist on the object like: { hasOwnProperty: false }
So a better way would be:
// Good
var obj = {"bar": "here bar desc"}
console.log(Object.prototype.hasOwnProperty.call(obj, "bar"));
// Best
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(obj, "bar"));

An ECMAScript 6 solution with reflection. Create a wrapper like:
/**
Gets an argument from array or object.
The possible outcome:
- If the key exists the value is returned.
- If no key exists the default value is returned.
- If no default value is specified an empty string is returned.
#param obj The object or array to be searched.
#param key The name of the property or key.
#param defVal Optional default version of the command-line parameter [default ""]
#return The default value in case of an error else the found parameter.
*/
function getSafeReflectArg( obj, key, defVal) {
"use strict";
var retVal = (typeof defVal === 'undefined' ? "" : defVal);
if ( Reflect.has( obj, key) ) {
return Reflect.get( obj, key);
}
return retVal;
} // getSafeReflectArg

Showing how to use this answer
const object= {key1: 'data', key2: 'data2'};
Object.keys(object).includes('key1') //returns true
We can use indexOf as well, I prefer includes

You need to use the method object.hasOwnProperty(property). It returns true if the object has the property and false if the object doesn't.

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).
const object1 = {};
object1.property1 = 42;
console.log(object1.hasOwnProperty('property1'));
// expected output: true
console.log(object1.hasOwnProperty('toString'));
// expected output: false
console.log(object1.hasOwnProperty('hasOwnProperty'));
// expected output: false
Know more

Don't over-complicate things when you can do:
var isProperty = (objectname.keyname || "") ? true : false;
It Is simple and clear for most cases...

A Better approach for iterating on object's own properties:
If you want to iterate on object's properties without using hasOwnProperty() check,
use for(let key of Object.keys(stud)){} method:
for(let key of Object.keys(stud)){
console.log(key); // will only log object's Own properties
}
full Example and comparison with for-in with hasOwnProperty()
function Student() {
this.name = "nitin";
}
Student.prototype = {
grade: 'A'
}
let stud = new Student();
// for-in approach
for(let key in stud){
if(stud.hasOwnProperty(key)){
console.log(key); // only outputs "name"
}
}
//Object.keys() approach
for(let key of Object.keys(stud)){
console.log(key);
}

Can you access a property of an object as a value in javascript?

var obj = { 'first':'1st', 'second':'2nd', 'third':'3rd', ...}
I know that accessing the value of an object's property is...
obj.first || obj["first"] //returns '1st' as a value
What I'm trying to figure out is how to access the property as a value without iterating through the whole var obj to make a new var that has the property switched with the value. It would just be a whole new obj.
Is this possible? Have I given enough context? I would appreciate any help.

Since an object is, well, an Object, you can pull the whole object keys into an Array, and then pull the one which is right for you..
Using the Object.keys() method you can access to the keys of your object, then, using the Array.indexOf() method you will pull which is right for you.
A Short Example
function getKey(obj, key) {
var arr = Object.keys(obj);
var result = '';
if (arr.indexOf(key) !== -1) {
var index = arr.indexOf(key);
result = arr[index];
return result;
}
return null;
}
var obj = {
'first': '1st',
'second': '2nd',
'third': '3rd'
}
console.log(getKey(obj, 'first'));
See:
MDN Object.keys() Ref.
A CodePen example

If i understand correctly you want to bind an objects property to another object's property so that when o1.first changes o2.first changes automatically. That would be possible with arranging the setter of o1.first property. A simple way to achieve this with object litereals would be like
var o1 = {
_first: "1st",
second: "2nd",
third: "3rd",
get first() {
return this._first
},
set first(v) {
this._first = v;
o2.first = v;
}
},
o2 = {first:""};
console.log(o1) // Object {_first: "1st", second: "2nd", third: "3rd"}
console.log(o2) // Object {first: ""}
o1.first = yes;
console.log(o1) // Object {_first: "yes", second: "2nd", third: "3rd"}
console.log(o2) // Object {first: "yes"}
o1["first"] = "once again"; // "once again"
console.log(o2["first"]); // "once again"
For more information on JS object getters and setters have a look at https://robertnyman.com/javascript/javascript-getters-setters.html

Javascript creating a normal variable fallback if object property does not exist

So now I nailed down the basics of javascript and I'm ready to get into the more intermediate arts of coding with "style". I'm trying to write easy maintainable code. The idea is to make a function work even if one of the object properties in use is not available by creating fallbacks. Problem is if I access the properties often then I would have to create ternary conditionals that create a simple for each accessed property. Right now you can see I'm only accessing object.a. I could of course store all the accessing of properties:
Idea 1
var a = (object.hasOwnProperty(a) ? object.a : a)
var b ...
var c ...
idea 2:
var a = (object['a'] ? object.a : a)
idea 3:
var a = object['a'] ? object.a : a
Idea 3:
(object.hasOwnProperty(a) ? var a = object.a : var a = 1);
Idea 4:
Switch statements?
At last:
object = {
// a: 1,
b: 2,
c: 3,
}
// normal vars in case one of the properties do not exist
var a = 1,
b = 2,
c = 3;
function x(){
var a = 1;
object.a * 10
if (object.a == not exist || (object.b == not exist|| (object.c == not exist)
then treat all non existing object properties accessed to as normal variables by doing:
convert object.a --> a
{

The ternary operator is a great way to do this. It allows you the ability to evaluate any expression you like to determine whether your candidate value is appropriate or whether you should use a fallback.
Examples...
// Use fallback if candidate is not "truthy"
var result = obj.a ? obj.a : "fallback";
// Use fallback if candidate is undefined
var result = obj.a !== undefined ? obj.a : "fallback";
// Use fallback if candidate is not defined on the object (whether or not it exists in the prototype chain)
var result = obj.hasOwnProperty(a) ? obj.a : "fallback";
You need to decide what condition you'd like to use a fallback value. Once you decide, wrap it in a function. Or make several similar functions which use different conditions.
Here's a function which checks to see if the candidate value is undefined and returns a fallback value.
function getOrDefault(candidate, fallback) {
if (typeof candidate === "undefined") {
return fallback;
}
return candidate;
}
// Example 1
var b = "alternate value";
var obj = { foo: "value" };
var result = getOrDefault(obj.a, b);
// result -> "alternate value";
// Example 2
var b = "alternate value";
var obj = { a: false };
var result = getOrDefault(obj.a, b);
// result -> false;
Also worth looking into is lodash's get function. Allows you to check for the existence of a property (even deeply nested properties) and allows you to specify a fallback.

ES6 provides nice facilities for doing what you want. The simplest example is
var { a = 1, b = 2 } = obj;
This uses destructuring assignment with defaults. The property a on obj is retrieved and assigned to variable a, but it if doesn't exist, a takes on the value 1.

Expanding on my comments about an "extend" or "merge" function, let's take a look at the following (from here):
var extend = function ( defaults, options ) {
var extended = {};
var prop;
for (prop in defaults) {
if (Object.prototype.hasOwnProperty.call(defaults, prop)) {
extended[prop] = defaults[prop];
}
}
for (prop in options) {
if (Object.prototype.hasOwnProperty.call(options, prop)) {
extended[prop] = options[prop];
}
}
return extended;
};
// Your "fallback" object values
var defaults = {
a: 1,
b: 2
};
// Your target object
var myObject = {
b: 4,
c: 8
};
// one line to override a set of defaults, producing a final object
console.log(extend(defaults, myObject)); // Object { a: 1, b: 4, c: 8 }

Conditionally adding properties to JavaScript object

Is there a more concise or readable way of doing this?
var foo={a:111,c:333, somePropertyThatShouldntBeAdded:'xxx'};
var myobj={x:1,y:2,z:3};
if(foo.a){myobj.a=foo.a;}
if(foo.b){myobj.b=foo.b;}
if(foo.c){myobj.c=foo.c;}
EDIT. Context why I am doing this is below.
var obj={value:this.text,css:{color:'#929292',margin:'1px 0px'}};
if(this.width){obj.css.width=this.width;}
if(this.type){obj.type=this.type;}
if(this.id){obj.id=this.id;}
var input=$('<input/>',obj)

You could use a simple loop-based approach:
var foo={a:111,c:333, somePropertyThatShouldntBeAdded:'xxx'};
var myobj={x:1,y:2,z:3};
['a','b','c'].forEach(function(key) {
if(key in foo) {
myobj[key] = foo[key];
}
});
Notice how I used the in keyword. Your current solution will not work if the value of a property is (e.g.) false or 0.
Additionaly, to get better solutions, provide some context: why are you conditionally copying properties? Perhaps you can avoid this to begin with.

With the introduction of the spread operator in ES2018 you can do something like this.
const original = {
first: null,
second: 'truthy'
};
const conditionalObject = {
...( original.first && { first: original.first }),
...( original.second && { second: original.second })
};
In the conditionalObject we first check if the property we want to add from the original is truthy, if it is we spread that property with its value into the new object.
If the property from the original object is falsy, the && short circuits and the property is never spread into the new object.
You can read a more detailed explanation here
The new conditionalObject will look like this
{
second: 'truthy'
}

You can use the ternary operator like this:
myobj.a = foo.a ? foo.a : undefined;
Though its not exactly the same as the if statement you have, because you'll have {a: undefined} instead of {}. The difference would show up if you ever enumerated the keys of your object.
Edit:
#hindmost has a good suggestion in the comments. You could improve it further with underscore.js:
_.extend(myobj, _.pick(foo, ['a', 'b', 'c']));

You could use jQuery's extend, then delete the properties you don't want:
function extendExcept(sourceObject, targetObject, except) {
var target = $.extend(sourceObject, targetObject);
except.forEach(function(key) {
delete target[key];
});
return target;
}
You could call it like:
var foo={a:111,c:333, somePropertyThatShouldntBeAdded:'xxx'};
var myobj={x:1,y:2,z:3};
myobj = extendExcept(foo, myobj, ["somePropertyThatShouldntBeAdded"]);
Working Example

var foo = {
a: 111,
c: 333,
somePropertyThatShouldntBeAdded: 'xxx'
};
var myObj = {
x: 1,
y: 2,
z: 3
};
for(var key in foo) {
if(key === 'somePropertyThatShouldntBeAdded') {
continue;
}
myObj[key] = foo[key];
}

method returning more length then the properties

i have a code snippet below but i dont know why the condition is not true??
Object.prototype.keys = function(){
var keys = [];
for ( var i in this )
keys.push( i );
return keys;
};
var obj = { a: 1, b: 2, c: 3 };
log( obj.keys().length == 3, "We should only have 3 properties." );
obj have only three properties but the returning me flase while i have also write the this statment before return
log(keys)
//outpue a,b,c
but i dont know why it is flase and the length is 4?

You are getting back the additional property "keys" which you set on the Object type.. which now all Objects have.
It would be the same as if you set the function on your object
var obj = { a: 1, b: 2, c: 3, keys: function(){ ... } };
4 Properties
Delegate the keys function to another class.. or just make it a function in your JS rather than a member function of your object. Don't attach they keys function with Object.prototype.. since appending it to all objects makes it incorrect for all objects.

For insure that your object has direct property only but not inherited then you can use hasOwnproperty() something like this.
for ( var i in this ) {
if(this.hasOwnProperty(i)){
//--use this--^ to insure that your object has direct property not inherited.
keys.push(i);
}
}
// returns keys[a, b, c]