JavaScript quirk 8: array-like objects (original) (raw)

[This post is part of a series on JavaScript quirks.]

Some objects in JavaScript look like arrays, but aren’t. They are called array-like. This blog post looks at what exactly that means and how to best work with those objects.

Array-like objects

An array-like object

• has: indexed access to elements and the property length that tells us how many elements the object has.
• does not have: array methods such as push, forEach and indexOf.

Two examples of array-like objects is the result of the DOM method

document.getElementsByClassName()

(many DOM methods return array-like objects) and the special variable

arguments

[1]. You can determine the number of arguments via

arguments.length

And you can access a single argument, e.g. read the first argument:

arguments[0]

Array methods, however, have to be borrowed. You can do that, because most of those methods are generic.

Generic methods

A generic method does not require

this

to be an array, it only requires

this

to have

length

and indexed element access. Normally, you invoke a method

m

on an array

arr

as follows.

arr.m(arg0, arg1, ...)

All functions have a method

call

that allows you to perform the above invocation differently:

Array.prototype.m.call(arr, arg0, arg1, ...)

The first argument of

call

is the value for

this

that

m

receives (in this case,

arr

). Because we access

m

directly and not via

arr

, we can now hand any

this

to that method. For example,

arguments

:

Array.prototype.m.call(arguments, arg0, arg1, ...)

Examples

Let’s continue with a concrete example. The following function

printArgs

logs all arguments that it receives.

function printArgs() {
    Array.prototype.forEach.call(arguments,
        function (arg, i) {
            console.log(i+'. '+arg);
        });
}

We have used method

forEach

generically.

printArgs

in use:

> printArgs()
> printArgs('a')
0. a
> printArgs('a', 'b')
0. a
1. b

You can even apply generic methods to ordinary objects:

> var obj = {};
> Array.prototype.push.call(obj, 'a');
1
> obj
{ '0': 'a', length: 1 }

In the above case, property

length

did not exist and was automatically created, with the initial value zero.

Converting an array-like object to an array

Sometimes the best way to work with an array-like object is to convert it to an array. That can also be done via a generic method:

Array.prototype.slice.call(arguments)

Compare: to create a copy of an array

arr

, you make the method call

arr.slice()

Reference

1. JavaScript quirk 5: parameter handling