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Asynchronous vs Deferred JavaScript

There are 3 ways how JavaScript can be loaded to a Website. The normal way, using the async attribute or the defer attribute.


<script src="/assets/js/bootstrap.min.js"></script>

In the normal way the parser stops, fetches the JavaScript files, executes it and continues with parsing the rest of the document.

This behavior is therefore render-blocking and can delay the overall page load if many and/or large resources need to be fetched.


<script async src="/assets/js/bootstrap.min.js"></script>

In the asynchronous way, the async attribute need to be added to the <script> tag. This way, the script is fetched asynchronous without blocking the parser. Once the script is fetched, the parsing is paused to execute the script.


<script defer src="/assets/js/bootstrap.min.js"></script>

In the deferred way, you need, like in the async way, add an attribute (defer) to the <script> tag. In this way, the script is fetched in the same way as in the asynchronous one, but doesn’t get executed until the parser has finished.


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