ExtJS is a JavaScript framework from Sencha for building Rich Internet Applications. It boasts one of the largest libraries of pre-built modular UI components.

Since version 5.0, Sencha has advocated the use of Model-View-ViewModel (MVVM) architecture on its platform. It also maintains support for Model-View-Controller (MVC) architecture which was the primary architecture style supported up through version 4.x.

Additionally, Sencha has focused on outfitting ExtJS with mobile-centric and responsive web application capabilities. Its former Sencha Touch framework has been integrated with ExtJS since version 6.0 with efforts to combine the customer bases and consolidate redundancies in the new combined framework.