Question

So, why does this result in 0 and how do I find the actual size?

var array = [];
array["foo"] = "bar";
array["bar"] = "foo";

document.write(array.length);

Answer 1

Since that is a object, which is comprised of properties, and take the form of key/value pairs (conceptually similar to an associative array, or a hash table) you will have to do something like this:

Object.size = function(obj) {
    var size = 0, key;
    for (key in obj) {
        if (obj.hasOwnProperty(key)) size++;
    }
    return size;
};

var array = [];
array["foo"] = "bar";
array["bar"] = "foo";

var size = Object.size(array);

Demo: http://jsfiddle.net/gBG34/