JavaScript: The Definitive Guide, 6th Edition

Array-Like Objects

As we’ve seen, JavaScript arrays have some special features that other objects do not have:

• The length property is automatically updated as new elements are added to the list.

• Setting length to a smaller value truncates the array.

• Arrays inherit useful methods from Array.prototype.

• Arrays have a class attribute of “Array”.

These are the features that make JavaScript arrays distinct from regular objects. But they are not the essential features that define an array. It is often perfectly reasonable to treat any object with a numeric length property and corresponding non-negative integer properties as a kind of array.

These “array-like” objects actually do occasionally appear in practice, and although you cannot directly invoke array methods on them or expect special behavior from the length property, you can still iterate through them with the same code you’d use for a true array. It turns out that many array algorithms work just as well with array-like objects as they do with real arrays. This is especially true if your algorithms treat the array as read-only or if they at least leave the array length unchanged.

The following code takes a regular object, adds properties to make it an array-like object, and then iterates through the “elements” of the resulting pseudo-array:

var a = {};  // Start with a regular empty object

// Add properties to make it "array-like"
var i = 0;
while(i < 10) {
    a[i] = i * i;
    i++;
}
a.length = i;

// Now iterate through it as if it were a real ...