ExtJS is a popular JavaScript library that has a wide variety of widgets, grids, graphing components, layout components, and more. With release 4.0, ExtJS incorporated a model, view, controller style of application architecture to their libraries. Although it is free for open source development, ExtJS requires a license for commercial use. It is popular with development teams that are building web-enabled desktop replacements, as its look and feel is comparable to normal desktop applications. ExtJS, by default, ensures that each application or component will look and feel exactly the same, no matter which browser it is run in, and it requires little or no need for CSS or HTML.

The ExtJS team, however, has not released an official TypeScript declaration file for ExtJS, despite much community pressure. Thankfully, the wider TypeScript community has come to the rescue, beginning with Mike Aubury. He wrote a small utility program to generate declaration files from the ExtJS documentation (https://github.com/zz9pa/extjsTypescript).

Whether this work influenced the current version of the ExtJS definitions on DefinitelyTyped remains to be seen, but the original definitions from Mike Aubury and the current version from brian428 on DefinitelyTyped are very similar.