java script


Introduction to JavaScript

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JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.

JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Mozilla, Firefox, Netscape, and Opera.

What You Should Already Know

Before you continue you should have a basic understanding of the following:

  • HTML / XHTML

If you want to study these subjects first, find the tutorials on our Home page.

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

The Real Name is ECMAScript

JavaScript’s official name is “ECMAScript”. The standard is developed and maintained by the ECMA organisation.

ECMA-262 is the official JavaScript standard. The standard is based on JavaScript (Netscape) and JScript (Microsoft).

The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996.

The development of ECMA-262 started in 1996, and the first edition of was adopted by the ECMA General Assembly in June 1997.

The standard was approved as an international ISO (ISO/IEC 16262) standard in 1998.

The development of the standard is still in progress.

The HTML <script> tag is used to insert a JavaScript into an HTML page.

Examples

Write text with Javascript
The example demonstrates how to use JavaSript to write text on a web page.

Write HTML with Javascript
The example demonstrates how to use JavaScript to write HTML tags on a web 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>

Try it yourself.

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.

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>

JavaScripts in the body section will be executed WHILE the page loads.

JavaScripts in the head section will be executed when CALLED.

Examples

Head section
Scripts that contain functions go in the head section of the document. Then we can be sure that the script is loaded before the function is called.

Body section
Execute a script that is placed in the body section.

External script
How to access an external script.

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>

Using an External JavaScript

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 src="xxx.js"></script>
</head>
<body>
</body>

JavaScript Statements

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JavaScript is a sequence of statements to be executed by the browser.

JavaScript Statements

A JavaScript statements is a command to the browser. The purpose of the command is to tell the browser what to do.

This JavaScript statement tells the browser to write “Hello Dolly” to the web page:

document.write("Hello Dolly");

It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the web.

The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement. Because of this you will often see examples without the semicolon at the end.

Note: Using semicolons makes it possible to write multiple statements on one line.

JavaScript Code

JavaScript code (or just JavaScript) is a sequence of JavaScript statements.

Each statement is executed by the browser in the sequence they are written.

This example will write a header and two paragraphs to a web page:

<script type="text/javascript">
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>

Try it yourself.

JavaScript Blocks

JavaScript statements can be grouped together in blocks.

Blocks start with a left curly bracket {, and ends with a right curly bracket }.

The purpose of a block is to make the sequence of statements execute together.

This example will write a header and two paragraphs to a web page:

<script type="text/javascript">
{
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
}

JavaScript Comments

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JavaScript comments can be used to make the code more readable.

JavaScript Comments

Comments can be added to explain the JavaScript, or to make it more readable.

Single line comments start with //.

This example uses single line comments to explain the code:

<script type="text/javascript">
// This will write a header:
document.write("<h1>This is a header</h1>");
// This will write two paragraphs:
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>

Try it yourself.

JavaScript Multi-Line Comments

Multi line comments start with /* and end with */.

This example uses a multi line comment to explain the code:

<script type="text/javascript">
/*
The code below will write
one header and two paragraphs
*/
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>

Try it yourself.

Using Comments to Prevent Execution

In this example the comment is used to prevent the execution of a single code line:

<script type="text/javascript">
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
//document.write("<p>This is another paragraph</p>");
</script>

Try it yourself.

In this example the comments is used to prevent the execution of multiple code lines:

<script type="text/javascript">
/*
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
*/
</script>

Try it yourself.

Using Comments at the End of a Line

In this example the comment is placed at the end of a line:

<script type="text/javascript">
document.write("Hello"); // This will write "Hello" 
document.write("Dolly"); // This will write "Dolly" 
</script>
 
</script>

Try it yourself.

The example above is not very useful. It just demonstrates the use of a block. Normally a block is used to group statements together in a function or in a condition (where a group of statements should be executed if a condition is met).

You will learn more about funct

</html>

Note: Remember to place the script exactly where you normally would write the script!

</html>

The two forward slashes at the end of comment line (//) is the JavaScript comment symbol. This prevents JavaScript from executing the –> tag.

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