There is a flexibility given to include JavaScript code anywhere in an HTML Document. However, the most preferred ways to include JavaScript in an HTML file are as follows:
• Script in <head>...</head> section.
• Script in <body>...</body> section.
• Script in <body>...</body> and <head>...</head> sections.
• Script in an external file and then include in <head>...</head> section.
Script in <head>...</head> section-
Now you want to have a script run on some event, such as when a user clicks somewhere, then you will place that script in the head as follows.
<html>
<head>
<script type= “text/JavaScript”>
function thismessage()
{
alert(“Hello World”);
}
</script>
</head>
<body>
Click here for the result
<input type= “button” onclick= “thismessage()” value= “This message” />
</body>
</html>
JavaScript in <body>...</body> segment-
If you require a script to run as the page loads so that the script generates content in the page, then the script goes in the <body> portion of the document. In this case, you would not have any function defined using JavaScript. Take a look at the following code.
<html>
<head>
</head>
<body>
<script type= “text/JavaScript”>
document.write(“Hello World”);
</script>
<p>This is web page body </p>
</body>
</html>
Script in <body>...</body> and <head>...</head> segment-
You can put your JavaScript code in <head> and <body> section altogether.
<html>
<head>
<script type= “text/JavaScript”>
function thismessage() {
alert(“Hello World!”);
}
</script>
</head>
<body>
<script type=”text/JavaScript”>
document.write(“Hello World!”);
</script>
<input type= “button” onclick= “thismessage()” value= “This Message” />
</body>
</html>
External file script and then include in <head>...</head> section-
As you begin to work more comprehensively with JavaScript, you will be likely to find that there are cases where you are reusing identical JavaScript code on multiple pages of a site. You are not restricted to be maintaining identical code in multiple HTML files.
The script tag provides a mechanism to allow you to store JavaScript in an external file and then include it into your HTML files.
Here is an example to show how you can include an external JavaScript file in your HTML code using script tag and its src attribute.
<html>
<head>
<script type= “text/JavaScript” src= “filename.js” > </script>
</head>
<body>
.....
</body>
</html>
To use JavaScript from an external file source, you need to write all your JavaScript source code in a simple text file with the extension “.js” and then include that file as shown above.
For example, you can keep the following content in filename.js file and then you can use message() function in your HTML file after including the filename.js file.
function message() {
alert(“Hello World”);
}