JavaScript Where to….?
JavaScript
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 and event.
JavaScript
in the body section will be executed WHILE the page loads.
JavaScript
in the head section will be executed when CALLED.
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.
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.
<html>
<head>
<script type=”text/javascript”>
….
</script>
</head>
Example:
<html>
<head>
<script type=”text/javascript”>
Function message()
{
Alert(“this alert box was called with the onload event”)
}
</script>
</head>
<body onload=”message()”>
</body>
</html>
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>
<script type=”text/javascript”>
….
</script>
Example:
<html>
<head>
</head>
<body>
<script type=”text/javascript”>
Document.write(“This message is written when the page
loads”)
</script>
</body>
</html>
Scripts in both the body and the head Section:
You can
place a unlimited number of scripts in your document, so you can have scripts
in both the body and the head section.
|
|
<head>
<script type=”text/javascript”>
….
</script>
</head>
<body>
<script type=”text/javascript”>
….
</script>
</body>
Using an External JavaScript
Sometime
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 Guidelines
Some other
important things to know when scripting with JavaScript.
JavaScript in Case Sensitive
A function
named “myfunction” is not the same as “myFunction” and a variable named “myVar”
is not the same as “myvar”.
JavaScript
is case sensitive_ therefore watch your capitalization closely when you create
or cal variable, objects and functions.
White Space
JavaScript
ignores extra spaces. You can add white space to your script to make it more
readable. The following lines are equivalent:
name=”Hege”
name= “Hege”
Break up a Code Line
You can
break up a code line within a text string with a backslash. The example beow
will be displayed property:
Document.write(“Hello\
World”)
However, you cannot break up a code line like this:
Document.write\
(“Hello World!”)
Comments
You can add comments to your script by using two slashes//:
//this is a comment
Document.write(“Hello World!”)
Or by using/*and*? (this create a multi-line comment block):
/* This is a comment
Block. It contains
Several lines */
Document.write(“Hello World!”)
