Objects in JavaScript

JavaScript has a much more limited knowledge of the world than you and

I. The objects it knows are all figments of computer memory, but those

figments are your concern when trying to design functional, interactive

Web pages.

JavaScript identifies its objects the same way you do, via a hierarchy

of progressively more specific objects. JavaScript knows that a browser

window, for example, has certain characteristics, such as size and

resize capabilities. The language knows how to make browser windows do

certain things, such as close. JavaScript knows the characteristics

(which it calls properties) and capabilities (which it calls methods)

of a whole bunch of objects that have to do with browsers and the

presentation of HTML documents. Collectively, these objects make up the

Document Object Model (DOM).

The syntax for referring to properties and methods of objects may seem

a little bit unusual at first. JavaScript identifies objects by a

series of keywords separated by periods. Here's an example:

window.frames[1].document.form1.textfield1

Don't panic. That hierarchy refers to a text field contained in a form,

which is a part of the document in frame 1 of the browser window. See?

Not too hard.

Once you've used the correct JavaScript syntax to identify the object

you want to work with, you can go on to specify the property you want

to inspect or the method you want to employ. Let's say you want to

refer to the value property of that form element, which contains the

contents of the text field in this case. In JavaScript, you'd say this:

window.frames[1].document.form.value

The property just gets tacked onto the end of the object specification.

When you refer to methods, you have to put parentheses after the

method's name. The two parentheses are there to hold parameters that

the method needs in order to do its stuff. You can sometimes leave them

empty. In other cases, they should hold numbers, strings of characters

or the URL of a needed resource that resides somewhere on the Internet.

Fun fact: There's a difference between an argument and a parameter,

even though the words frequently are used as if they were

interchangeable. Parameters are the variable -- like definitions that

appear in function declarations. Arguments are the values that are

actually sent to the function when it is called. I have to go now; Star

Trek is on.

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