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

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 2

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 3

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 4

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 5

Simply use this to get the length:

Object.keys(myObject).length

Answer 6

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 7

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 8

We can find the length of Object by using:

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

Answer 9

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 10

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 11

Use:

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

Answer 12

<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 13

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 14

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 15

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 16

@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 17

The simplest way is like this:

Object.keys(myobject).length

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

Answer 18

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 19

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 20

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 21

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 22

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

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

// o/p 3

Answer 23

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 24

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 25

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 26

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

Answer 27

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 28

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 29

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