$A
$A(iterable) -> actualArray
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 Array
s 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.
Examples
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'));
paras.each(Element.hide);
$(paras.last()).show();
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 hide
instance method we first must make sure all the target elements are extended:
$A(document.getElementsByTagName('p')).map(Element.extend).invoke('hide');
Want to display your arguments easily? Array
features a join
method, but the arguments
value that exists in all functions does not inherit from Array
. So, the tough way, or the easy way?
// The hard way...
function showArgs() {
alert(Array.prototype.join.call(arguments, ', '));
}
// The easy way...
function showArgs() {
alert($A(arguments).join(', '));
}