Number of elements in a javascript object

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.