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;

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.

---

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

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;
};

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;

Answer 5

Simply use this to get the length:

Object.keys(myObject).length

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".

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

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. ;)

Answer 9

We can find the length of Object by using:

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

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);

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

Use:

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

Answer 15

<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 16

The simplest way is like this:

Object.keys(myobject).length

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

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

@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 19

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

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

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

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

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>

Answer 24

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

console.log(Object.keys(myObject).length)

// o/p 3

Answer 25

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 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

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/

Answer 28

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

Answer 29

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()

Answer 30

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.

Answer 31

Below is a version of James Coglan's answer in CoffeeScript for those who have abandoned straight JavaScript :)

Object.size = (obj) ->
  size = 0
  size++ for own key of obj
  size

Answer 32

Property

Object.defineProperty(Object.prototype, 'length', {
    get: function () {
        var size = 0, key;
        for (key in this)
            if (this.hasOwnProperty(key))
                size++;
        return size;
    }
});

Use

var o = {a: 1, b: 2, c: 3};
alert(o.length); // <-- 3
o['foo'] = 123;
alert(o.length); // <-- 4

Answer 33

With the ECMAScript 6 in-built Reflect object, you can easily count the properties of an object:

Reflect.ownKeys(targetObject).length

It will give you the length of the target object's own properties (important).

Reflect.ownKeys(target)

Returns an array of the target object's own (not inherited) property keys.

Now, what does that mean? To explain this, let's see this example.

function Person(name, age){
  this.name = name;
  this.age = age;
}

Person.prototype.getIntro= function() {
  return `${this.name} is ${this.age} years old!!`
}

let student = new Person('Anuj', 11);

console.log(Reflect.ownKeys(student).length) // 2
console.log(student.getIntro()) // Anuj is 11 years old!!

You can see here, it returned only its own properties while the object is still inheriting the property from its parent.

For more information, refer this: Reflect API

Answer 34

Try: Object.values(theObject).length

const myObject = new Object();
myObject["firstname"] = "Gareth";
myObject["lastname"] = "Simpson";
myObject["age"] = 21;
console.log(Object.values(myObject).length);

Answer 35

Like most JavaScript problems, there are many solutions. You could extend the Object that for better or worse works like many other languages' Dictionary (+ first class citizens). Nothing wrong with that, but another option is to construct a new Object that meets your specific needs.

function uberject(obj){
    this._count = 0;
    for(var param in obj){
        this[param] = obj[param];
        this._count++;
    }
}

uberject.prototype.getLength = function(){
    return this._count;
};

var foo = new uberject({bar:123,baz:456});
alert(foo.getLength());

Answer 36

Simple one liner:

console.log(Object.values({id:"1",age:23,role_number:90}).length);

Answer 37

Simple solution:

  var myObject = {};      // ... your object goes here.

  var length = 0;

  for (var property in myObject) {
    if (myObject.hasOwnProperty(property)){
      length += 1;
    }
  };

  console.log(length);    // logs 0 in my example.

Answer 38

Here you can give any kind of varible array,object,string

function length2(obj){
    if (typeof obj==='object' && obj!== null){return Object.keys(obj).length;}
   //if (Array.isArray){return obj.length;}
    return obj.length;

}

Answer 39

The solution work for many cases and cross browser:

Code

var getTotal = function(collection) {

    var length = collection['length'];
    var isArrayObject =  typeof length == 'number' && length >= 0 && length <= Math.pow(2,53) - 1; // Number.MAX_SAFE_INTEGER

    if(isArrayObject) {
        return collection['length'];
    }

    i= 0;
    for(var key in collection) {
        if (collection.hasOwnProperty(key)) {
            i++;
        }
    }

    return i;
};

Data Examples:

// case 1
var a = new Object();
a["firstname"] = "Gareth";
a["lastname"] = "Simpson";
a["age"] = 21;

//case 2
var b = [1,2,3];

// case 3
var c = {};
c[0] = 1;
c.two = 2;

Usage

getLength(a); // 3
getLength(b); // 3
getLength(c); // 2

Answer 40

Object.keys does not return the right result in case of object inheritance. To properly count object properties, including inherited ones, use for-in. For example, by the following function (related question):

var objLength = (o,i=0) => { for(p in o) i++; return i }

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

var child = Object.create(myObject);
child["sex"] = "male";

var objLength = (o,i=0) => { for(p in o) i++; return i }

console.log("Object.keys(myObject):", Object.keys(myObject).length, "(OK)");
console.log("Object.keys(child)   :", Object.keys(child).length, "(wrong)");
console.log("objLength(child)     :", objLength(child), "(OK)");

Answer 41

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 42

 vendor = {1: "", 2: ""}
 const keysArray = Object.keys(vendor)
 const objectLength = keysArray.length
 console.log(objectLength)
 Result 2

Answer 43

I had a similar need to calculate the bandwidth used by objects received over a websocket. Simply finding the length of the Stringified object was enough for me.

websocket.on('message', data => {
    dataPerSecond += JSON.stringify(data).length;
}

Answer 44

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

var size = JSON.stringify(myObject).length;

document.write(size);

JSON.stringify(myObject)