How to find length of literal array?

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

First off, the length is 0 because the number of items in array is 0.

When you do this syntax array["foo"] = "bar" you're creating a property called foo that has a value of bar. The length is still 0 since you have not added anything to the Array, you've just set new properties on the Array

Working version here: http://jsfiddle.net/VyjJD/3/

var array = [];

array["foo"] = "bar";
array["bar"] = "foo";
array.bin = function() { return "bin"; };

array[0] = "bar";
array[1] = "foo";
array[2] = "bin";
array[3] = "bat";

var props = 0;

for (key in array)
    props++;

document.write(array.length + "<br />" 
           + props + "<br />" 
           + (array.foo == array["foo"]) + "<br />" 
           + array.bar + "<br />"
           + array.bin());

Notice that array.length = 4 but props = 7 which is all of the properties and the number of items in the Array.

Answer 2

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/

Answer 3

You are setting a property on the array, not giving it a new element. Arrays can receive arbitrary properties, just like any other Javascript object. For instance:

var foo = [];
foo.bar = 'foobar';
console.log(foo.bar); // outputs 'foobar'
console.log(foo); // outputs [] -- empty array

To add items to an array, use Array.push:

var foo = [];
foo.push('foobar');
console.log(foo); // outputs ['foobar']

If you want key=>value pairs, use an object instead:

var foo = {};
foo['bar'] = 'foobar';
console.log(foo); // outputs {bar: 'foobar'}

Answer 4

basically when you do

array["foo"] = "bar"

it just adds some more attributes to the array, which is an object. In javascript, array.foo and array['foo'] means the same.