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

It comes down to that, or keeping count when you add and delete properties from the object.

Answer 2

unfortunately there isn't really a good way to know the number of properties in an object without iterating over them. The following is fairly simple though:

function countProps(obj) {
    var l = 0;
    for (p in obj) l++;
    return l;
}

var bob = { a: 1, b: 2};

alert(countProps(bob));

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

Answer 4

Firefox supports the count property. In other browsers the following solution should suffice:

function count(obj) {

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

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

    return c;

}