Technologies

 


Where To Use Javascript

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>
</html>

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

JavaScript Variables

A variable is a "container" for information you want to store.

Examples

Variable
Variables are used to store data. This example will show you how.

Variables

A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value.
Rules for variable names:

  • Variable names are case sensitive
  • They must begin with a letter or the underscore character

IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!

Declare a Variable

You can create a variable with the var statement:


var strname = some value

You can also create a variable without the var statement:


strname = some value

Assign a Value to a Variable

You can assign a value to a variable like this:


var strname = "Hege"

Or like this:


strname = "Hege"

The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege".

Lifetime of Variables

When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.
If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.
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