Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

3.5 Objects

An

object is a collection of named values. These
named values are usually referred to as

properties of the object. (Sometimes they are
called fields of the object, but this usage can be confusing.) To
refer to a property of an object, you refer to the object, followed
by a period and the name of the property. For example, if an object
named image
has properties named
width and height, we can refer
to those properties like this:

image.width
image.height

Properties of objects are, in many ways, just like JavaScript
variables; they can contain any type of data, including arrays,
functions, and other objects. Thus, you might see JavaScript code
like this:

document.myform.button

This code refers to the button property of an
object that is itself stored in the myform
property of an object named document.

As mentioned earlier, when a function value
is stored in a property of an object, that function is often called a
method, and the property name becomes the method name. To invoke a
method of an object, use the
. syntax to extract the function value from the
object, and then use the ( ) syntax to invoke that
function. For example, to invoke the write( )
method of the Document object, you can use code like this:

document.write("this is a test");

Objects in JavaScript have the ability
to serve as associative arrays -- that is, they can associate
arbitrary data values with arbitrary strings. When an object is used
in this way, a different syntax is generally required to access the
object's properties: a

string containing the name of the
desired property is enclosed within square brackets. Using this syntax,
we could access the properties of the
image object mentioned previously with
code like this:

image["width"]
image["height"]

Associative arrays are a powerful data type; they are useful for a
number of programming techniques. We'll learn more about
objects in their traditional and associative array usages in Chapter 8.

3.5.1 Creating Objects

As we'll see in
Chapter 8, objects are created by invoking special

constructor functions. For example, the
following lines all create new objects:

var o = new Object( );
var now = new Date( );
var pattern = new RegExp("\\sjava\\s", "i");

Once you have created an object of your own, you can use and set its
properties however you desire:

var point = new Object( );
point.x = 2.3;
point.y = -1.2;

3.5.2 Object Literals

ECMAScript v3 defines (and JavaScript
1.2 implements) an object literal syntax that allows you to create an
object and specify its properties. An object literal (also called an
object initializer) consists of a comma-separated list of
colon-separated property/value pairs, all enclosed within
curly
braces. Thus, the object
point in the previous code could
also be created and initialized with this line:

var point = { x:2.3, y:-1.2 };

Object literals can also be
nested. For example:

var rectangle = { upperLeft: { x: 2, y: 2 },
lowerRight: { x: 4, y: 4}
};

Finally, the property
values used in object literals need
not be constant -- they can be arbitrary JavaScript
expressions:

var square = { upperLeft: { x:point.x, y:point.y },
lowerRight: { x:(point.x + side), y:(point.y+side) }};