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accept an optional
properties
argument. We will discuss this method further in
Chapter 8,
ECMAScript 5th Edition.
7.5.2 The
tddjs.extend
Method
Often we want to borrow behavior from one or more other objects to build the
functionality we're after. We've seen this a couple of times already. Remember the
arguments
object? It acts roughly like an array, but it is not a true array, and
as such, lacks certain properties we might be interested in. The
arguments
ob-
ject does, however, possess the most important aspects of an array: the
length
property, and numerical indexes as property names. These two aspects are enough
for most methods on
Array.prototype
to consider
arguments
an object
that “walks like a duck, swims like a duck, and quacks like a duck” and there-
fore is a duck (or rather, an array). This means that we can borrow methods from
Array.prototype
by calling them with
arguments
as
this
, as seen in
Listing 7.50.
Listing 7.50
Borrowing from
Array.prototype
"test arguments should borrow from Array.prototype":
function () {
function addToArray() {
var args = Array.prototype.slice.call(arguments);
var arr = args.shift();
return arr.concat(args);
}
var result = addToArray([], 1, 2, 3);
assertEquals([1, 2, 3], result);
}
The example borrows the
slice
function and calls it on the
arguments
object. Because we don't give it any other arguments, it will return the whole array,
but the trick is now we've effectively converted
arguments
to an array, on which
we can call the usual array methods.
Remember in Chapter 5,
Functions,
we illustrated implicit binding of
this
by copying a function from one object to another. Doing so causes both objects
to share the same function object, so it's a memory efficient way to share behavior.
Listing 7.51 shows an example.
