We've seen plenty of examples of JavaScript's built-in objects and
how they're used: Window, Image, Document, Form, and a bevy of other objects. Remember: An object is just a special kind of data, made
up of properties and methods. Properties are the things that an
object knows about itself, while methods are the things that the object
knows how to do.
For example, the Window object in JavaScript has properties,
like window.status, which can be read and set. By setting the status property of the Window object to a string, it makes that string appear in the
window's status bar. Window also has methods, like window.focus(), which brings a window forward. If this stuff is unfamiliar to you, you should probably read Part I, Day 5 for more details.
Objects provide a great way to organize information. First, we deal with real-life objects all the time. A monkey, for example, is an object. Its properties include height, weight, furriness, and sassiness. All monkeys have the same set of properties. It's the values of those properties that makes one monkey different from another. Monkeys also have
methods, or things they do, like play(), eat(), sleep().
Objects are also handy because they allow you to preserve
the "sense" of words. Take the notion of focus() in JavaScript, for example. Focus(), appropriately enough,
brings things into focus. When focus() is applied to a window, it brings the window forward. Blur does the reverse — try it.
open little_window
Click on the link below to see how focus brings the little window forward.
little_window.focus();
However, when you apply focus() to a text box, it puts a typing cursor inside the box.
window.document.the_form.the_first_text.focus();
window.document.the_form.the_second_text.focus();
Spooky, huh? JavaScript knows that focus
does one thing for a window and something else for a textbox. The idea of
focus is the same in both cases, so it makes sense to call it the same thing.
But it also makes sense that focus() does slightly different things for two different objects. This notion of having one method name do different things for different
objects is called "polymorphism," and it's fundamental to object-oriented programming.
Just as you can have the same method do different things for different objects, you can also have the same property mean different things in different objects.
The length property, for example, works for both strings and arrays.
The string.length returns the length of a string, while array.length returns the number of things in the array. Both properties are called length, but they mean slightly different things in the
context of different objects.
Objects also make it easy to cut and paste code together. If you have a good library of objects, you can just copy an entire object and plunk it into whatever script you need. This is a lot harder when you don't have your code organized into clear objects.
Since objects are so handy, JavaScript gives you the ability to create your own.
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