How to get object length [duplicate]

Is there any built-in function that can return the length of an object?

For example, I have a = { 'a':1,'b':2,'c':3 } which should return 3. If I use a.length it returns undefined.

It could be a simple loop function, but I'd like to know if there's a built-in function?

There is a related question (Length of a JSON object) - in the chosen answer the user advises to transform object into an array, which is not pretty comfortable for my task.

Solution 1:

For browsers supporting Object.keys() you can simply do:

Object.keys(a).length;

Otherwise (notably in IE < 9), you can loop through the object yourself with a for (x in y) loop:

var count = 0;
var i;

for (i in a) {
    if (a.hasOwnProperty(i)) {
        count++;
    }
}

The hasOwnProperty is there to make sure that you're only counting properties from the object literal, and not properties it "inherits" from its prototype.

Solution 2:

This should do it:

Object.keys(a).length

However, Object.keys is not supported in IE8 and below, Opera and FF 3.6 and below.

Live demo: http://jsfiddle.net/simevidas/nN84h/

Solution 3:

Can be done easily with $.map():

var len = $.map(a, function(n, i) { return i; }).length;

Solution 4:

Have you taken a look at underscore.js (http://underscorejs.org/docs/underscore.html)? It's a utility library with a lot of useful methods. There is a collection size method, as well as a toArray method, which may get you what you need.

_.size({one : 1, two : 2, three : 3});
=> 3