Modules were not supported in JavaScript until ECMAScript 2015, because JavaScript was initially designed to add small bits of interactivity to web pages, not to cater for building full-fledged applications. When developers wanted to use a library or framework, they'd simply add <script> tags somewhere inside the HTML, and that library would be loaded when the page was loaded. However, this is not ideal as the scripts must be loaded in the correct order. For example, Bootstrap (a UI framework) depends on jQuery (a utility library), so we must manually check that the jQuery script is added first:
<!-- jQuery - this must come first --><script src="https://code.jquery.com/jquery-3.2.1.min.js"></script><!-- Bootstrap's JavaScript ...
