It is generally recommended to place your JavaScript files at the bottom of your HTML document.
This becomes especially crucial when utilizing traditional <script> tag files. Many browsers, including modern ones, will halt the UI thread and consequently the rendering process of your HTML markup while loading and executing JavaScript.
As a result, if you load a significant amount of JavaScript at the top of your page, users may experience slower page loading times since they will only see your complete markup after all scripts have been loaded and executed.
To exacerbate this issue, most browsers do not download JavaScript files in parallel. For instance:
<script type="javascript" src="/path/file1.js"></script>
<script type="javascript" src="/path/file2.js"></script>
<script type="javascript" src="/path/file3.js"></script>
Your browser will
- load file1.js
- execute file1.js
- load file2.js
- execute file2.js
- load file3.js
- execute file3.js
During this process, both the UI thread and the rendering process are blocked.
Some browsers like Chrome have started loading script files in parallel, mitigating this problem to some extent.
An alternative approach to address this issue is by using dynamic script tag insertion. This involves loading just one JavaScript file via a <script> tag, which then dynamically generates additional <script> tags and inserts them into the markup. This method operates asynchronously and offers improved performance.