How do I get number of fields in JavaScript object?

Question

I'm trying to use JavaScript object as an associative array and everything was well until I needed to get number of entries that are stored in it. What is the easiest and most elegant way to do that? All I can think of is to run for each loop or jQuery $.each function and just see how much iterations it would do but that looks like an an awful thing to do.

Answer 1

This worked for me:

Object.keys(obj).length

Answer 2

Old Firefox supports the __count__ property. Newer environments support ES5's Object.keys. For older environments we have to fallback to just iterating over the object and counting manually (ugh!):

function count(obj) {

    if (obj.__count__ !== undefined) { // Old FF
        return obj.__count__;
    }

    if (Object.keys) { // ES5 
        return Object.keys(obj).length;
    }

    // Everything else:

    var c = 0, p;
    for (p in obj) {
        if (obj.hasOwnProperty(p)) {
            c += 1;
        }
    }

    return c;

}

Answer 3

I believe this has been answered on here before, and/or googling can lead you in the right direction. Nevertheless let me point out that one of the biggest gotchas if you use a loop in the form of:

for (var attr in obj) { ....

Is that the object could be cluttered with properties you did not necessarily add, and the standard solution to this as I recall is to additionally use the test for hasOwnProperty, see https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Object/hasOwnProperty