Wednesday, November 19, 2008

Introduction to JavaScript

JavaScript is a programming language that can be included on web pages to make them more interactive. You can use it to check or modify the contents of forms, change images, open new windows and write dynamic page content. You can even use it with CSS to make DHTML (Dynamic HyperText Markup Language). This allows you to make parts of your web pages appear or disappear or move around on the page. JavaScripts only execute on the page(s) that are on your browser window at any set time. When the user stops viewing that page, any scripts that were running on it are immediately stopped. The only exception is a cookie, which can be used by many pages to pass information between them, even after the pages have been closed.

Before we go any further, let me say; JavaScript has nothing to do with Java. If we are honest, JavaScript, originally nicknamed LiveWire and then LiveScript when it was created by Netscape, should in fact be called ECMAscript as it was renamed when Netscape passed it to the ECMA for standardisation.

JavaScript is a client side, interpreted, object oriented, high level scripting language, while Java is a client side, compiled, object oriented high level language. Now after that mouthful, here's what it means.

Client side
Programs are passed to the computer that the browser is on, and that computer runs them. The alternative is server side, where the program is run on the server and only the results are passed to the computer that the browser is on. Examples of this would be PHP, Perl, ASP, JSP etc.
The program is passed as source code with all the programming language visible. It is then converted into machine code as it is being used. Compiled languages are converted into machine code first then passed around, so you never get to see the original programming language. Java is actually dual half compiled, meaning it is half compiled (to 'byte code') before it is passed, then executed in a virtual machine which converts it to fully compiled code just before use, in order to execute it on the computer's processor. Interpreted languages are generally less fussy about syntax and if you have made mistakes in a part they never use, the mistake usually will not cause you any problems.

This is a little harder to define. Scripting languages are often used for performing repetitive tasks. Although they may be complete programming languages, they do not usually go into the depths of complex programs, such as thread and memory management. They may use another program to do the work and simply tell it what to do. They often do not create their own user interfaces, and instead will rely on the other programs to create an interface for them. This is quite accurate for JavaScript. We do not have to tell the browser exactly what to put on the screen for every pixel, we just tell it that we want it to change the document, and it does it. The browser will also take care of the memory management and thread management, leaving JavaScript free to get on with the things it wants to do.
High level
Written in words that are as close to english as possible. The contrast would be with assembly code, where each command can be directly translated into machine code.
Object oriented
See the section on 'object oriented programming' for details.

How is JavaScript constructed

The basic part of a script is a variable, literal or object. A variable is a word that represents a piece of text, a number, a boolean true or false value or an object. A literal is the actual number or piece of text or boolean value that the variable represents. An object is a collection of variables held together by a parent variable, or a document component.

The next most important part of a script is an operator. Operators assign literal values to variables or say what type of tests to perform.

The next most important part of a script is a control structure. Control structures say what scripts should be run if a test is satisfied.

Functions collect control structures, actions and assignments together and can be told to run those pieces of script as and when necessary.

The most obvious parts of a script are the actions it performs. Some of these are done with operators but most are done using methods. Methods are a special kind of function and may do things like submitting forms, writing pages or displaying messages.

Events can be used to detect actions, usually created by the user, such as moving or clicking the mouse, pressing a key or resetting a form. When triggered, events can be used to run functions.

Lastly and not quite so obvious is referencing. This is about working out what to write to access the contents of objects or even the objects themselves.

As an example, think of the following situation. A person clicks a submit button on a form. When they click the button, we want to check if they have filled out their name in a text box and if they have, we want to submit the form. So, we tell the form to detect the submit event. When the event is triggered, we tell it to run the function that holds together the tests and actions. The function contains a control structure that uses a comparison operator to test the text box to see that it is not empty. Of course we have to work out how to reference the text box first. The text box is an object. One of the variables it holds is the text that is written in the text box. The text written in it is a literal. If the text box is not empty, a method is used that submits the form.



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