October 11, 2010

Javascript Introduction

What You Should Already Know?
Before you continue you should have a basic understanding of the following:
HTML / XHTML
What is JavaScript?
• JavaScript was designed to add interactivity to HTML pages
• JavaScript is a scripting language
• A scripting language is a lightweight programming language
• A JavaScript consists of lines of executable computer code
• A JavaScript is usually embedded directly into HTML pages
• JavaScript is an interpreted language (means that scripts execute without
preliminary compilation)
• Everyone can use JavaScript without purchasing a license


Are Java and JavaScript the Same?
NO!
Java and Javascript are two completely different languages in both concept and design!
Java (developed by Sun Microsystems) is a powerful and much more complex
programming language ­ in the same category as C and C++.
What can a JavaScript Do?
• JavaScript gives HTML designers a programming tool ­ HTML authors are
normally not programmers, but JavaScript is a scripting language with a very
simple syntax! Almost anyone can put small "snippets" of code into their HTML
pages
• JavaScript can put dynamic text into an HTML page ­ A JavaScript statement
like this: document.write("<h1>" + name + "</h1>")
 can write a variable text into
an HTML page
• JavaScript can react to events ­ A JavaScript can be set to execute when
something happens, like when a page has finished loading or when a user clicks
on an HTML element
• JavaScript can read and write HTML elements ­ A JavaScript can read and
change the content of an HTML element
• JavaScript can be used to validate data ­ A JavaScript can be used to validate
form data before it is submitted to a server. This saves the server from extra
processing
• JavaScript can be used to detect the visitor's browser ­ A JavaScript can be
used to detect the visitor's browser, and ­ depending on the browser ­ load another
page specifically designed for that browser
• JavaScript can be used to create cookies ­ A JavaScript can be used to store and
retrieve information on the visitor's computer



JavaScript How To ...
The HTML the <script> tag is used to insert a JavaScript into an HTML page.
How to Put a JavaScript Into an HTML Page
<html>


<body>


<script type="text/javascript">


document.write("Hello World!");


</script>


</body>


</html>
The code above will produce this output on an HTML page:
Hello World!


Example Explained
To insert a JavaScript into an HTML page, we use the

<script> tag. Inside the

<script>
tag we use the "type=" attribute to define the scripting language.
So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and
ends: <html>

<body>


<script type="text/javascript">


...


</script>


</body>


</html>

The word document.write is a standard JavaScript command for writing output to a
page.
By entering the document.write command between the <script> and </script> tags, the
browser will recognize it as a JavaScript command and execute the code line. In this case
the browser will write Hello World! to the page:
<html>


<body>


<script type="text/javascript">


document.write("Hello World!");


</script>


</body>


</html>

Hot Tip: If we had not entered the the <script> tag, the browser would have treated the
document.write("Hello World!") command as pure text, and just write the entire line on
the page.
HTML Comments to Handle Simple Browsers
Browsers that do not support JavaScript will display JavaScript as page content.
To prevent them from doing this, and as a part of the JavaScript standard, the HTML
comment tag can be used to "hide" the JavaScript. Just add an HTML comment tag <!­­

before the first JavaScript statement, and a ­­>
(end of comment) after the last JavaScript
statement.


<html>


<body>


<script type="text/javascript">


<!­­


document.write("Hello World!");


//­­>


</script></body>


</html>

The two forward slashes at the end of comment line (//) is the JavaScript comment
symbol. This prevents JavaScript from executing the --> tag.
JavaScript Where To ...
JavaScripts in the body section will be executed WHILE the page loads.
JavaScripts in the head section will be executed when CALLED.


Where to Put the JavaScript?
JavaScripts in a page will be executed immediately while the page loads into the browser.
This is not always what we want. Sometimes we want to execute a script when a page
loads, other times when a user triggers an event.
Scripts in the head section: Scripts to be executed when they are called, or when an
event is triggered, go in the head section. When you place a script in the head section,
you will ensure that the script is loaded before anyone uses it.
<html>


<head>


<script type="text/javascript">


....


</script>


</head>

Scripts in the body section: Scripts to be executed when the page loads go in the body
section. When you place a script in the body section it generates the content of the page.


<html>


<head>


</head>


<body>


<script type="text/javascript">


....


</script>


</body>

Scripts in both the body and the head section: You can place an unlimited number of
scripts in your document, so you can have scripts in both the body and the head section.


<html>


<head>


<script type="text/javascript">


....


</script>


</head>


<body>


<script type="text/javascript">


....


</script>


</body>.


This are Sum basics of Javascript.
we will continue with more info on this subject In next post.

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