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APPENDIX

Employing the

call()

method of

Function

causes the function context to

be set to whatever object is passed as the first parameter to

call()

--in this

case,

o2

g

. In this example, the function acts like a method to

o2

, even

though it has no association whatsoever--even as a property--with

o2

.

As with

call()

, using the

apply()

method of

Function

sets the function con-

text to whatever object is passed as the first parameter

h

. The difference

between these two methods only becomes significant when parameters are
passed to the function (which we didn't do in this example for simplicity).

This example page clearly demonstrates that the function context is determined
on a per invocation basis and that a single function can be called with any object
acting as its context. It's, therefore, probably never correct to say that a function is
a method of an object. It's much more correct to state the following:

A function f acts as a method of object o when o serves as the function context of the
invocation of f.

As a further illustration of this concept, consider the effect of adding the follow-
ing statement to our example:

alert(o1.identifyMe.call(o3));

Even though we reference the function as a property of

o1

, the function context

for this invocation is

o3

, further emphasizing that it's not how a function is

declared but how it's invoked that determines its function context.

When using jQuery commands and functions that employ callbacks, this

proves to be an important concept. We saw this concept in action early on (even if
you didn't realize it at the time) in section 2.3.3 where we supplied a callback
function to the

filter()

method of

$

and that function was sequentially invoked

with each element of the wrapped set serving as its function context in turn.

Now that we understand how functions can act as methods of objects, let's turn

our attention to another advanced function topic that will play an important role
in effective usage of jQuery--closures.

A.2.4 Closures

To page authors coming from a traditional

OO

or procedural programming back-

ground, closures are often an odd concept to grasp; whereas, to those with a func-
tional programming background, they're a familiar and cozy concept. For the
uninitiated, let's answer the question: What are closures?

Stated as simply as possible, a closure is a

Function

instance coupled with the

local variables from its environment that are necessary for its execution.