Length of a JavaScript object

Question

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;

• cloudfeet: @neitony - that's kinda true, but so many people are used to PHP's "associative array" that they might assume it means "ordered associative map", when JS objects are in fact unordered.
• Andrew Koster: In the above example, myObject.length is undefined, at least in a browser environment. That's why it isn't valid, @AdrianM
• Sebastian Simon: Variants of Object.{keys, values, entries}(obj).length have now been mentioned a total of 38 times in 16 answers plus in the comments of this question, and another 11 times in 7 deleted answers. I think that’s enough now.

Answer 1

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.

---

• vsync: @Tres - your code can be broken if someone would come and overide the 'size' property without knowing you declared it already somewhere in the code, so it's always worth checking if it's already defined
• Tres: @vsync You are very correct. One should always implement necessary sanity checks :)
• Muhammad Umer: Why is everyone ignoring this: Object.keys(obj).length
• Chris Hayes: @MuhammadUmer Probably because that method didn't even exist when this answer was written. Even today, using it will probably require a polyfill for old browsers.
• Lukas Liesis: @stonyau IE8, IE9, IE10 are dead browsers that don't get support from Microsoft. IE8, IE9, IE10 user gets notification from Microsoft, that they use old, unsupported browser and should expect that stuff will not work for them. support.microsoft.com/en-us/kb/3123303

Answer 2

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

• vsync: why not? from what I know, it is a standard: developer.mozilla.org/en/JavaScript/Reference/Global_Objects/…
• aeosynth: It's not a universally implemented method, but you can check which browser supports it with this table.
• mdup: time to switch to firefox = unfortunately, you switching doesn't mean your website's users will...
• John Dvorak: @ripper234 no IE support = time to polyfill
• albanx: @ripper234 who cares about IE, no one should care about IE unstandards, only standards. users wanna use IE then they will not navigate my website I do not care any more. developers should not polyfill not standards made by ie "developers"

Answer 3

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;
};

• tokyovariable: _.size() was the perfect solution for my Meteor project, which has underscore.js support.
• jbolanos: I use underscore and this post just reminded me that I'm not using it enough in this project. If you handle objects, you should have underscore.js available.
• Rayjax: Underscore > (all -['lo-dash'])
• anwar: underscorejs, things that should be under js :)
• ripper234: @Babydead to distinguish real properties of the object vs inherited ones. developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

Answer 4

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;

• John Slegers: Object.keys() returns an array that contains the names of only those properties that are enumerable. If you want an array with ALL properties, you should use Object.getOwnPropertyNames() instead. See developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

Answer 5

Simply use this to get the length:

Object.keys(myObject).length

• Patata: please explain how your answer differs from stackoverflow.com/a/6700/8632727
• Barry Michael Doyle: Good, it's always better to name your variables according to what they actually are. Makes your code more readable to other devs.
• Yunnosch: Before the "myArray" -> "myObject" edit, this was identical to the second-highest upvoted answer.
• Alberto Perez: Same as past responses.
• Zsolt Meszaros: Same as the linked answer, but without any explanation.

Answer 6

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".

• Lajos Mészáros: Will not work, because it will count methods too, that are added through prototype.

Answer 7

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

• Muhammad Umer: @PatrickRoberts keys method doesn't return properties from prototype chain. So why the need for hasOwnProperty here. Also getOwnProperty will return hidden properties, as length is in array etc.

Answer 8

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. ;)

• Emeka Mbah: I think this is the best solution as it doesn't require looping with 'for' which could be costly if the array is big

Answer 9

We can find the length of Object by using:

const myObject = {};
console.log(Object.values(myObject).length);

• JSG: Theoretically, his would be slower than the "keys" method if you have long values as it is directly accessing the values then counting them.

Answer 10

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++;

Answer 11

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);

• Panos Theof: I think that is not safe. For example it cannot have an element with a key of "length", the statement a1["length"] = "Hello world"; fails to store the entry. Also the statement a1["hasOwnProperty"] = "some prop"; totaly breaks the function
• Ally: @PanosTheof I don't think you'd want it to store the value if you used the length property, any code that used it would have to ensure it did not try and store against length, but I guess that would be the same if it was a standard array too. Overriding hasOwnProperty on any object would most likely produce an undesired result.

Answer 12

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

Answer 13

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

Answer 14

Using the Object.entries method in order to retrieve the object 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

Answer 15

Use:

var myArray = new Object();
myArray["firstname"] = "Gareth";
myArray["lastname"] = "Simpson";
myArray["age"] = 21;
obj = Object.keys(myArray).length;
console.log(obj)

Answer 16

<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;

Answer 17

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.

Answer 18

The simplest way is like this:

Object.keys(myobject).length

Where myobject is the object of what you want the length of.

• Pang: This appears to be just a repeat of this existing answer.
• Mystical: @Pang agreed, and it does not provide any further context like other answers do.

Answer 19

@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.

Answer 20

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

• chim: This is a great answer, however I can't find any documentation for this keys function. So I can't be confident on the cross browser support.
• chim: Unfortunately, this is not as good an answer as I first thought! It turns out that the keys function is only available in the chrome and firefox web consoles. If you put this code in to a script then it will fail with Uncaught ReferenceError: keys is not defined
• chim: stackoverflow.com/questions/32372685/…
• Peter Mortensen: How is this different from aeosynth's answer?

Answer 21

var myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;

1. Object.values(myObject).length
2. Object.entries(myObject).length
3. Object.keys(myObject).length

• Siluveru Kiran Kumar: Which one is faster among the above 3.
• tdjprog: Object.values(myObject).length
• Siluveru Kiran Kumar: how can we say that it Object.values(myObject).length is faster is there any example Thanks, @tdjprog
• tdjprog: just try this in console:var myObject = {}; for (let i=0; i<10000000; i++) myObject[i] = i;
• Siluveru Kiran Kumar: why Object.values(myObject).length faster, where as Object.entries(myObject).length not giving output even after sometime what is the reason here ? Thanks @tdjprog

Answer 22

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--; }}
}

Answer 23

Here are some methods:

const myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
//first method
console.log(Object.entries(myObject).length); //3
//second method
let len = 0;
for(let i in myObject)len++;
console.log(len); //3

Answer 24

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>

• hodgef: The OP has not asked for a AngularJS version. This is not a valid answer to the question.

Answer 25

const myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
console.log(Object.keys(myObject).length)
// o/p 3

• Peter Mortensen: What does "o/p 3" mean?
• Peter Mortensen: An explanation of this answer would be in order.
• Peter Mortensen: Also, how is it different from previous answers, e.g. shaheb's answer?
• Sebastian Simon: @PeterMortensen "o/p" probably means "output".
• Soura Ghosh: o/p means output

Answer 26

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.

Answer 27

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:

1. 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.
2. 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.

Answer 28

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/

• Dziad Borowy: Object.prototype - bad idea.
• Mohamad: @tborychowski can you please explain why?
• Dziad Borowy: here's one article: bit.ly/1droWrG. I'm not saying it mustn't be done, only that you need to know all the repercussions before you do this.
• Sebastian Simon: If you’re going to extend built-in prototypes or polyfill a property (i.e. monkey-patch), please do it correctly: for forward compatibility, check if the property exists first, then make the property non-enumerable so that the own keys of constructed objects aren’t polluted. For methods use actual methods. My recommendation: follow these examples which demonstrate how to add a method that behaves as closely as possible like built-in methods.

Answer 29

You can always do Object.getOwnPropertyNames(myObject).length to get the same result as [].length would give for normal array.

• John Slegers: Object.keys() returns an array that contains the names of only those properties that are enumerable. If you want an array with ALL properties, you should use Object.getOwnPropertyNames() instead. See developer.mozilla.org/en/docs/Web/JavaScript/Reference/…
• Peter Mortensen: How is this different from aeosynth's answer?

Answer 30

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()

• Peter Mortensen: How is this different from aeosynth's answer?
• Mystical: It's because it shows how to make it as a function and a global object method (more object oriented and uses some form of encapsulation); however aeosynth's answer doesn't.
• Sebastian Simon: If you’re going to extend built-in prototypes or polyfill a property (i.e. monkey-patch), please do it correctly: for forward compatibility, check if the property exists first, then make the property non-enumerable so that the own keys of constructed objects aren’t polluted. For methods use actual methods. My recommendation: follow these examples which demonstrate how to add a method that behaves as closely as possible like built-in methods.
• Sebastian Simon: Also, how is writing a method "more efficient"?