Number of elements in a javascript object

Question

Although JS implementations might keep track of such a value internally, there's no standard way to get it.

In the past, Mozilla's Javascript variant exposed the non-standard __count__, but it has been removed with version 1.8.5.

For cross-browser scripting you're stuck with explicitly iterating over the properties and checking hasOwnProperty():

function countProperties(obj) {
    var count = 0;

    for(var prop in obj) {
        if(obj.hasOwnProperty(prop))
            ++count;
    }

    return count;
}

In case of ECMAScript 5 capable implementations, this can also be written as (Kudos to Avi Flax)

function countProperties(obj) {
    return Object.keys(obj).length;
}

Keep in mind that you'll also miss properties which aren't enumerable (eg an array's length).

If you're using a framework like jQuery, Prototype, Mootools, $whatever-the-newest-hype, check if they come with their own collections API, which might be a better solution to your problem than using native JS objects.

To do this in any ES5-compatible environment

Object.keys(obj).length

(Browser support from here)
(Doc on Object.keys here, includes method you can add to non-ECMA5 browsers)

if you are already using jQuery in your build just do this:

$(yourObject).length

It works nicely for me on objects, and I already had jQuery as a dependancy.

function count(){
    var c= 0;
    for(var p in this) if(this.hasOwnProperty(p))++c;
    return c;
}

var O={a: 1, b: 2, c: 3};

count.call(O);