Where to Put Your JavaScript in a Web Page
Scripts 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">
some statements
</script>
</head>
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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">
some statements
</script>
</body>
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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">
some statements
</script>
</head>
<body>
<script type="text/javascript">
some statements
</script>
</body>
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How to Run an External JavaScript
Sometimes you might want to run the same script on several pages, without
writing the script on each and every page.
To simplify this you can write a
script in an external file, and save it with a .js file extension, like this:
document.write("This script is external")
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Save the external file as xxx.js.
Note: The external script cannot contain the <script> tag
Now you can call this script, using the "src" attribute, from any of your pages:
<html>
<head>
</head>
<body>
<script src="xxx.js"></script>
</body>
</html>
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Remember to place the script exactly where you normally would write
the script.
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.
<html>
<head>
<script type="text/javascript">
function message()
{
alert("This alert box was called with the onload event")
}
</script>
</head>
<body>
</body>
</html>
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Body section
Execute a script that is placed in the body section.
<html>
<body>
<script type="text/javascript">
document.write("This message is written when the page loads")
</script>
</body>
</html>
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External script
How to access an external script.
<html>
<head>
<script src="xxx.js"></script>
</head>
<body>
In this case, the script is in an external script file called "xxx.js".
</body>
</html>
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