JavaScript quirk 8: array-like objects

[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


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