Tutorials
Intro to Javascript
JavaScript is the most popular scripting language on the web, and he is my friend (ok, just kidding). JavaScript works in all major browsers, from Internet Explorer to Firefox, from Chrome to Opera, and Safari. Designed to add interactivity to HTML pages, JavaScript was developed as an interpreted language, which means that scripts execute without preliminary compilation.
JavaScript, not to be confused with Java, as they are two completely different languages in concept and design, is as powerful as C and C++ in programming language. JavaScript provides HTML authors with a programming tool, even without prior experience. The small "snippets" of code making this process simple. 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. It can also can react to events, read and write HTML elements , validate data--even before it is submitted to a server. JavaScript can also be used to detect the visitor's browser, and depending on the browser - load another page specifically designed for that browser. All this among many other exceptionally useful things.
To insert JavaScript into an HTML page, use the <script> tag. Inside the <script> tag, 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 document.write command 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>
Note: If we had not entered 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. Try it yourself
How 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 should 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, like this:
<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.
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: cripts 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>
Basically, JavaScripts in the body section will be executed WHILE the page loads, while JavaScripts in the head section will be executed when called.
Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.
To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.
Note: The external script cannot contain the <script> tag!
To use the external script, point to the .js file in the "src" attribute of the <script> tag:
<html>
<head>
<script type="text/javascript" src="xxx.js"></script>
</head>
<body>
</body>
</html>
Note: Remember to place the script exactly where you normally would write the script!
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