This is a quick tip for iterating over arrays and other objects with a length property. It’s particularly suitable for sparse arrays, without having to worry whether any methods have been added to the prototype chain.

Dean Edwards recently described a pretty cool technique for enumerating javascript objects, but (currently) it’s not appropriate for sparse arrays. In case you’re unfamiliar with sparse arrays (or I’m using the wrong term), in the language of the Mozilla documentation for Array.forEach, a sparse array is one that’s not “dense”. For example:

// create a three-element array
var array = ['hope', 'empire', 'jedi'];
// add a fourth
array[987654321] = 'menace'; 

// because array length is now 987654322
// this is a really inefficient way of iterating:
for (var i = 0; i < array.length; i++) {
    ... do something ...

Iterating over this array using the for loop shown above would mean the loop is executed hundreds of millions of times. That’s terribly inefficient for an array containing so few elements. A more efficient way is to use for (property in object), with an if condition to filter out irrelevant properties:

for (var property in object) {
    if (String(property >>> 0) == property &&
            property >>> 0 != 0xffffffff) {
        ... do something ...

The if condition checks whether the property is an array index as defined by the language specification:

A property name P (in the form of a string value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232−1

That’s it.