utility $A

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$A(iterable) → Array

Accepts an array-like collection (anything with numeric indices) and returns its equivalent as an actual Array object. This method is a convenience alias of Array.from, but is the preferred way of casting to an Array.

The primary use of $A is to obtain an actual Array object based on anything that could pass as an array (e.g. the NodeList or HTMLCollection objects returned by numerous DOM methods, or the predefined arguments reference within your functions).

The reason you would want an actual Array is simple: Prototype extends Array to equip it with numerous extra methods, and also mixes in the Enumerable module, which brings in another boatload of nifty methods. Therefore, in Prototype, actual Arrays trump any other collection type you might otherwise get.

The conversion performed is rather simple: null, undefined and false become an empty array; any object featuring an explicit toArray method (as many Prototype objects do) has it invoked; otherwise, we assume the argument "looks like an array" (e.g. features a length property and the [] operator), and iterate over its components in the usual way.

When passed an array, $A makes a copy of that array and returns it.


The well-known DOM method document.getElementsByTagName() doesn't return an Array, but a NodeList object that implements the basic array "interface." Internet Explorer does not allow us to extend Enumerable onto NodeList.prototype, so instead we cast the returned NodeList to an Array:

var paras = $A(document.getElementsByTagName('p'));

Notice we had to use each and Element.hide because $A doesn't perform DOM extensions, since the array could contain anything (not just DOM elements). To use the Element#hide instance method we first must make sure all the target elements are extended:


Want to display your arguments easily? Array features a join method, but the arguments value that exists in all functions does not inherit from Array. So, the tough way, or the easy way?

// The hard way...
function showArgs() {
  alert(Array.prototype.join.call(arguments, ', '));
 // The easy way...
function showArgs() {
  alert($A(arguments).join(', '));

Aliased as: Array.from