External JavaScript File
You can place all your scripts into an external file (with a .js extension), then link to that file from within your HTML document. This is handy if you need to use the same scripts across multiple HTML pages, or a whole website.
To link to an external JavaScript file, you add a src attribute to your HTML script tag and specify the location of the external JavaScript file.
Linking to an external JavaScript file
<script type="text/javascript" src="external_javascript.js"></script>
Contents of your external JavaScript file
The code in your .js file should be no different to the code you would normally have placed in between the script tags. But remember, you don't need to create script tag again - it is already on the HTML page calling the external file!