How to efficiently count the number of keys/properties of an object in JavaScript?

What's the fastest way to count the number of keys/properties of an object? It it possible to do this without iterating over the object? i.e. without doing

var count = 0;
for (k in myobj) if (myobj.hasOwnProperty(k)) count++;

(Firefox did provide a magic __count__ property, but this was removed somewhere around version 4.)

Answers:

Answer

To do this in any ES5-compatible environment, such as Node, Chrome, IE 9+, Firefox 4+, or Safari 5+:

Object.keys(obj).length
Answer

You could use this code:

if (!Object.keys) {
    Object.keys = function (obj) {
        var keys = [],
            k;
        for (k in obj) {
            if (Object.prototype.hasOwnProperty.call(obj, k)) {
                keys.push(k);
            }
        }
        return keys;
    };
}

Then you can use this in older browsers as well:

var len = Object.keys(obj).length;
Answer

If you are using Underscore.js you can use _.size (thanks @douwe):
_.size(obj)

Alternatively you can also use _.keys which might be clearer for some:
_.keys(obj).length

I highly recommend Underscore, its a tight library for doing lots of basic things. Whenever possible they match ECMA5 and defer to the native implementation.

Otherwise I support @Avi's answer. I edited it to add a link to the MDC doc which includes the keys() method you can add to non-ECMA5 browsers.

Answer

The standard Object implementation (ES5.1 Object Internal Properties and Methods) does not require an Object to track its number of keys/properties, so there should be no standard way to determine the size of an Object without explicitly or implicitly iterating over its keys.

So here are the most commonly used alternatives:

1. ECMAScript's Object.keys()

Object.keys(obj).length; Works by internally iterating over the keys to compute a temporary array and returns its length.

  • Pros - Readable and clean syntax. No library or custom code required except a shim if native support is unavailable
  • Cons - Memory overhead due to the creation of the array.

2. Library-based solutions

Many library-based examples elsewhere in this topic are useful idioms in the context of their library. From a performance viewpoint, however, there is nothing to gain compared to a perfect no-library code since all those library methods actually encapsulate either a for-loop or ES5 Object.keys (native or shimmed).

3. Optimizing a for-loop

The slowest part of such a for-loop is generally the .hasOwnProperty() call, because of the function call overhead. So when I just want the number of entries of a JSON object, I just skip the .hasOwnProperty() call if I know that no code did nor will extend Object.prototype.

Otherwise, your code could be very slightly optimized by making k local (var k) and by using prefix-increment operator (++count) instead of postfix.

var count = 0;
for (var k in myobj) if (myobj.hasOwnProperty(k)) ++count;

Another idea relies on caching the hasOwnProperty method:

var hasOwn = Object.prototype.hasOwnProperty;
var count = 0;
for (var k in myobj) if (hasOwn.call(myobj, k)) ++count;

Whether this is faster or not on a given environment is a question of benchmarking. Very limited performance gain can be expected anyway.

Answer

If you are actually running into a performance problem I would suggest wrapping the calls that add/remove properties to/from the object with a function that also increments/decrements an appropriately named (size?) property.

You only need to calculate the initial number of properties once and move on from there. If there isn't an actual performance problem, don't bother. Just wrap that bit of code in a function getNumberOfProperties(object) and be done with it.

Answer

I'm not aware of any way to do this, however to keep the iterations to a minimum, you could try checking for the existance of __count__ and if it doesn't exist (ie not Firefox) then you could iterate over the object and define it for later use eg:

if (myobj.__count__ === undefined) {
  myobj.__count__ = ...
}

This way any browser supporting __count__ would use that, and iterations would only be carried out for those which don't. If the count changes and you can't do this, you could always make it a function:

if (myobj.__count__ === undefined) {
  myobj.__count__ = function() { return ... }
  myobj.__count__.toString = function() { return this(); }
}

This way anytime you reference myobj.__count__ the function will fire and recalculate.

Answer

As stated by Avi Flax https://stackoverflow.com/a/4889658/1047014

Object.keys(obj).length

will do the trick for all enumerable properties on your object but to also include the non-enumerable properties you can instead use the Object.getOwnPropertyNames. Here's the difference:

var myObject = new Object();

Object.defineProperty(myObject, "nonEnumerableProp", {
  enumerable: false
});
Object.defineProperty(myObject, "enumerableProp", {
  enumerable: true
});

console.log(Object.getOwnPropertyNames(myObject).length); //outputs 2
console.log(Object.keys(myObject).length); //outputs 1

console.log(myObject.hasOwnProperty("nonEnumerableProp")); //outputs true
console.log(myObject.hasOwnProperty("enumerableProp")); //outputs true

console.log("nonEnumerableProp" in myObject); //outputs true
console.log("enumerableProp" in myObject); //outputs true

As stated here this has the same browser support as Object.keys

However, in most cases, you might not want to include the nonenumerables in these type of operations, but it's always good to know the difference ;)

Answer

To iterate on Avi Flax answer Object.keys(obj).length is correct for an object that doesnt have functions tied to it

example:

obj = {"lol": "what", owo: "pfft"};
Object.keys(obj).length; // should be 2

versus

arr = [];
obj = {"lol": "what", owo: "pfft"};
obj.omg = function(){
    _.each(obj, function(a){
        arr.push(a);
    });
};
Object.keys(obj).length; // should be 3 because it looks like this 
/* obj === {"lol": "what", owo: "pfft", omg: function(){_.each(obj, function(a){arr.push(a);});}} */

steps to avoid this:

  1. do not put functions in an object that you want to count the number of keys in

  2. use a seperate object or make a new object specifically for functions (if you want to count how many functions there are in the file using Object.keys(obj).length)

also yes i used the _ or underscore module from nodejs in my example

documentation can be found here http://underscorejs.org/ as well as its source on github and various other info

And finally a lodash implementation https://lodash.com/docs#size

_.size(obj)

Answer

as answered above: Object.keys(obj).length

But: as we have now a real Map class in ES6, I would suggest to use it instead of using the properties of an object.

const map = new Map();
map.set("key", "value");
map.size; // THE fastest way
Answer

For those who have Underscore.js included in their project you can do:

_({a:'', b:''}).size() // => 2

or functional style:

_.size({a:'', b:''}) // => 2
Answer

From: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

Object.defineProperty(obj, prop, descriptor)

You can either add it to all your objects:

Object.defineProperty(Object.prototype, "length", {
    enumerable: false,
    get: function() {
        return Object.keys(this).length;
    }
});

Or a single object:

var myObj = {};
Object.defineProperty(myObj, "length", {
    enumerable: false,
    get: function() {
        return Object.keys(this).length;
    }
});

Example:

var myObj = {};
myObj.name  = "John Doe";
myObj.email = "[email protected]";
myObj.length; //output: 2

Added that way, it won't be displayed in for..in loops:

for(var i in myObj) {
     console.log(i + ":" + myObj[i]);
}

Output:

name:John Doe
email:[email protected]

Note: it does not work in < IE9 browsers.

Answer

How I've solved this problem is to build my own implementation of a basic list which keeps a record of how many items are stored in the object. Its very simple. Something like this:

function BasicList()
{
   var items = {};
   this.count = 0;

   this.add = function(index, item)
   {
      items[index] = item;
      this.count++;
   }

   this.remove = function (index)
   {
      delete items[index];
      this.count--;
   }

   this.get = function(index)
   {
      if (undefined === index)
        return items;
      else
        return items[index];
   }
}
Answer

You can use Object.keys(data).length to find the length of JSON object having key data

Answer

For those that have Ext JS 4 in their project you can do:

Ext.Object.getSize(myobj);

The advantage of this is that it'll work on all Ext compatible browsers (IE6-IE8 included), however, I believe the running time is no better than O(n) though, as with other suggested solutions.

Answer

Here are some performance tests for three methods;

https://jsperf.com/get-the-number-of-keys-in-an-object

Object.keys().length

20,735 operations per second

Very simple and compatible. Runs fast but expensive because it creates a new array of keys, which that then gets thrown away.

return Object.keys(objectToRead).length;

loop through the keys

15,734 operations per second

let size=0;
for(let k in objectToRead) {
  size++
}
return size;

Slightly slower, but nowhere near the memory usage, so probably better if you're interested in optimising for mobile or other small machines

Using Map instead of Object

953,839,338 operations per second

return mapToRead.size;

Basically, Map tracks its own size so we're just returning a number field. Far, far faster than any other method. If you have control of the object, convert them to maps instead.

Answer

If jQuery above does not work, then try

$(Object.Item).length
Answer

OP didn't specify if the object is a nodeList, if it is then you can just use length method on it directly. Example:

buttons = document.querySelectorAll('[id=button)) {
console.log('Found ' + buttons.length + ' on the screen'); 
Answer

You can use:

Object.keys(objectName).length; 

and

Object.values(objectName).length;
Answer

I don't think this is possible (at least not without using some internals). And I don't think you would gain much by optimizing this.

Answer

I try to make it available to all object like this:

Object.defineProperty(Object.prototype, "length", {
get() {
    if (!Object.keys) {
        Object.keys = function (obj) {
            var keys = [],k;
            for (k in obj) {
                if (Object.prototype.hasOwnProperty.call(obj, k)) {
                    keys.push(k);
                }
            }
            return keys;
        };
    }
    return Object.keys(this).length;
},});

console.log({"Name":"Joe","Age":26}.length) //returns 2
Answer

Google Closure has a nice function for this... goog.object.getCount(obj)

look at goog.Object Documentation

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