JavaScript code can be written inline (e.g, within HTML tags called event handlers),
in script blocks, and in external JavaScript files.
Script in and external file and then include in <head>...</head> section.
Script in <head>...</head> section
If you want to have a script run on some event,
such as on click of a button, then you will place that script inside the head tag.
For example,
<html>
<head>
<script type="text/javascript">
<!--
function HelloWorld() {
alert("Hello World")
}
//-->
</script>
</head>
<body>
<input type="button" onclick="HelloWorld()" value="Click Me" />
</body>
</html>
Script in <body>...</body> section
If you need a script to run as the page loads so that the script generates
content in the page, the script goes in the <body> portion of the
document.
For example,
<html>
<head>
</head>
<body>
<script type="text/javascript">
<!--
document.write("Hello World")
//-->
</script>
</body>
</html>
Script in and external file
The script tag provides a mechanism to allow you to store JavaScript in
an external file and then include it into your HTML files.
For example,
<html>
<head>
<script type="text/javascript" src="filename.js" ></script>
</head>
<body>
<p>Using external JavaScript file.<br/>
<input type="button" onclick="HelloWorld()" value="Click Me" />
</body>
</html>
you can keep following content in filename.js file and then you can use
HelloWorld function in your HTML file after including filename.js file:
function sayHello() {
alert("Hello World")
}
