Why do websites include parameters in their CSS and JavaScript link files?

Why Include Parameters in CSS and JavaScript Link Files?

Question:

When examining various website source codes, we notice that many sites pass parameters to the CSS and JavaScript link files. For example, in the Stack Overflow source code, we find the following script:

<script type="text/javascript" src="http://sstatic.net/js/master.js?v=55c7eccb8e19"></script>

Why is this parameter master.js?v=55c7eccb8e19 included?

Answer:

Parameters are included in CSS and JavaScript link files primarily to prevent caching.

Explanation:

When a client visits a website, their browser may cache the CSS and JavaScript files to improve performance on subsequent visits. However, if the CSS or JavaScript files are updated, the cached version may become outdated and prevent the client from seeing the latest changes.

By appending a unique parameter to the link file, such as ?v=55c7eccb8e19, the browser is forced to request the file from the server every time. This prevents it from using the cached version and ensures that the client always receives the latest version of the file.

This technique is particularly useful when deploying new versions of your application. By updating the parameter value, you can force clients to refresh their CSS and JavaScript files and load the changes immediately.

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