ExtJS is a JavaScript library for building rich internet applications. It’s compatible with major browsers and is used to build rich, sustainable web applications, fast.

One of the good thing about ExtJS is that it supports plugin. Another good thing is, ExtJS plugins doesn’t need any installation. What you have to do to use it is to just include the plugin in your HTML code the same way as you include other JavaScripts, as the following example. 

<script type="text/javascript" src="myextjsplugin.js"></script>

What’s important is the loading sequence. Depending on your code, and plugin, you might need to load the plugin before loading the core ExtJS libraries, but that just depends.