What are alternatives to ExtJS?

So what I'm looking for is a javascript framework I can use that has several UI controls. I have taken a look at jQuery but those controls are very basic compared to ExtJS. Are there any other competitive alternatives?

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Nothing compares to extjs in terms of community size and presence on StackOverflow. Despite previous controversy, Ext JS now has a GPLv3 open source license. Its learning curve is long, but it can be quite rewarding once learned. Ext JS lacks a Material Design theme, and the team has repeatedly refused to release the source code on GitHub. For mobile, one must use the separate Sencha Touch library.

Have in mind also that,

large JavaScript libraries, such as YUI, have been receiving less attention from the community. Many developers today look at large JavaScript libraries as walled gardens they don’t want to be locked into.

-- Announcement of YUI development being ceased

That said, below are a number of Ext JS alternatives currently available.

Leading client widget libraries

Blueprint is a React-based UI toolkit developed by big data analytics company Palantir in TypeScript, and "optimized for building complex data-dense interfaces for desktop applications". Actively developed on GitHub as of May 2019, with comprehensive documentation. Components range from simple (chips, toast, icons) to complex (tree, data table, tag input with autocomplete, date range picker. No accordion or resizer.

Blueprint targets modern browsers (Chrome, Firefox, Safari, IE 11, and Microsoft Edge) and is licensed under a modified Apache license.

Sandbox / demo • GitHub • Docs
Webix - an advanced, easy to learn, mobile-friendly, responsive and rich free&open source JavaScript UI components library. Webix spun off from DHTMLX Touch (a project with 8 years of development behind it - see below) and went on to become a standalone UI components framework. The GPL3 edition allows commercial use and lets non-GPL applications using Webix keep their license, e.g. MIT, via a license exemption for FLOSS. Webix has 55 UI widgets, including trees, grids, treegrids and charts. Funding comes from a commercial edition with some advanced widgets (Pivot, Scheduler, Kanban, org chart etc.). Webix has an extensive list of free and commercial widgets, and integrates with most popular frameworks (React, Vue, Meteor, etc) and UI components.

Skins look modern, and include a Material Design theme. The Touch theme also looks quite Material Design-ish. See also the Skin Builder.

Minimal GitHub presence, but includes the library code, and the documentation (which still needs major improvements). Webix suffers from a having a small team and a lack of marketing. However, they have been responsive to user feedback, both on GitHub and on their forum.

The library was lean (128Kb gzip+minified for all 55 widgets as of ~2015), faster than ExtJS, dojo and others, and the design is pleasant-looking. The current version of Webix (v6, as of Nov 2018) got heavier (400 - 676kB minified but NOT gzipped).

The demos on Webix.com look and function great. The developer, XB Software, uses Webix in solutions they build for paying customers, so there's likely a good, funded future ahead of it.

Webix aims for backwards compatibility down to IE8, and as a result carries some technical debt.

Wikipedia • GitHub • Playground/sandbox • Admin dashboard demo • Demos • Widget samples
react-md - MIT-licensed Material Design UI components library for React. Responsive, accessible. Implements components from simple (buttons, cards) to complex (sortable tables, autocomplete, tags input, calendars). One lead author, ~1900 GitHub stars.
kendo - jQuery-based UI toolkit with 40+ basic open-source widgets, plus commercial professional widgets (grids, trees, charts etc.). Responsive&mobile support. Works with Bootstrap and AngularJS. Modern, with Material Design themes. The documentation is available on GitHub, which has enabled numerous contributions from users (4500+ commits, 500+ PRs as of Jan 2015).

Well-supported commercially, claiming millions of developers, and part of a large family of developer tools. Telerik has received many accolades, is a multi-national company (Bulgaria, US), was acquired by Progress Software, and is a thought leader.

A Kendo UI Professional developer license costs $700 and posting access to most forums is conditioned upon having a license or being in the trial period.

[Wikipedia] • GitHub/Telerik • Demos • Playground • Tools
OpenUI5 - jQuery-based UI framework with 180 widgets, Apache 2.0-licensed and fully-open sourced and funded by German software giant SAP SE.

The community is much larger than that of Webix, SAP is hiring developers to grow OpenUI5, and they presented OpenUI5 at OSCON 2014.

The desktop themes are rather lackluster, but the Fiori design for web and mobile looks clean and neat.

Wikipedia • GitHub • Mobile-first controls demos • Desktop controls demos • SO
DHTMLX - JavaScript library for building rich Web and Mobile apps. Looks most like ExtJS - check the demos. Has been developed since 2005 but still looks modern. All components except TreeGrid are available under GPLv2 but advanced features for many components are only available in the commercial PRO edition - see for example the tree. Claims to be used by many Fortune 500 companies.

Minimal presence on GitHub (the main library code is missing) and StackOverflow but active forum. The documentation is not available on GitHub, which makes it difficult to improve by the community.
Polymer, a Web Components polyfill, plus Polymer Paper, Google's implementation of the Material design. Aimed at web and mobile apps. Doesn't have advanced widgets like trees or even grids but the controls it provides are mobile-first and responsive. Used by many big players, e.g. IBM or USA Today.
Ant Design claims it is "a design language for background applications", influenced by "nature" and helping designers "create low-entropy atmosphere for developer team". That's probably a poor translation from Chinese for "UI components for enterprise web applications". It's a React UI library written in TypeScript, with many components, from simple (buttons, cards) to advanced (autocomplete, calendar, tag input, table).

The project was born in China, is popular with Chinese companies, and parts of the documentation are available only in Chinese. Quite popular on GitHub, yet it makes the mistake of splitting the community into Chinese and English chat rooms. The design looks Material-ish, but fonts are small and the information looks lost in a see of whitespace.
PrimeUI - collection of 45+ rich widgets based on jQuery UI. Apache 2.0 license. Small GitHub community. 35 premium themes available.
qooxdoo - "a universal JavaScript framework with a coherent set of individual components", developed and funded by German hosting provider 1&1 (see the contributors, one of the world's largest hosting companies. GPL/EPL (a business-friendly license).

Mobile themes look modern but desktop themes look old (gradients).

Wikipedia • GitHub • Web/Mobile/Desktop demos • Widgets Demo browser • Widget browser • SO • Playground • Community
jQuery UI - easy to pick up; looks a bit dated; lacks advanced widgets. Of course, you can combine it with independent widgets for particular needs, e.g. trees or other UI components, but the same can be said for any other framework.
angular + Angular UI. While Angular is backed by Google, it's being radically revamped in the upcoming 2.0 version, and "users will need to get to grips with a new kind of architecture. It's also been confirmed that there will be no migration path from Angular 1.X to 2.0". Moreover, the consensus seems to be that Angular 2 won't really be ready for use until a year or two from now. Angular UI has relatively few widgets (no trees, for example).
DojoToolkit and their powerful Dijit set of widgets. Completely open-sourced and actively developed on GitHub, but development is now (Nov 2018) focused on the new dojo.io framework, which has very few basic widgets. BSD/AFL license. Development started in 2004 and the Dojo Foundation is being sponsored by IBM, Google, and others - see Wikipedia. 7500 questions here on SO.

Themes look desktop-oriented and dated - see the theme tester in dijit. The official theme previewer is broken and only shows "Claro". A Bootstrap theme exists, which looks a lot like Bootstrap, but doesn't use Bootstrap classes. In Jan 2015, I started a thread on building a Material Design theme for Dojo, which got quite popular within the first hours. However, there are questions regarding building that theme for the current Dojo 1.10 vs. the next Dojo 2.0. The response to that thread shows an active and wide community, covering many time zones.

Unfortunately, Dojo has fallen out of popularity and fewer companies appear to use it, despite having (had?) a strong foothold in the enterprise world. In 2009-2012, its learning curve was steep and the documentation needed improvements; while the documentation has substantially improved, it's unclear how easy it is to pick up Dojo nowadays.

With a Material Design theme, Dojo (2.0?) might be the killer UI components framework.

Wikipedia • GitHub • Themes • Demos • Desktop widgets • SO
Enyo - front-end library aimed at mobile and TV apps (e.g. large touch-friendly controls). Developed by LG Electronix and Apache-licensed on GitHub.
The radical Cappuccino - Objective-J (a superset of JavaScript) instead of HTML+CSS+DOM
Mochaui, MooTools UI Library User Interface Library. <300 GitHub stars.
CrossUI - cross-browser JS framework to develop and package the exactly same code and UI into Web Apps, Native Desktop Apps (Windows, OS X, Linux) and Mobile Apps (iOS, Android, Windows Phone, BlackBerry). Open sourced LGPL3. Featured RAD tool (form builder etc.). The UI looks desktop-, not web-oriented. Actively developed, small community. No presence on GitHub.
ZinoUI - simple widgets. The DataTable, for instance, doesn't even support sorting.
Wijmo - good-looking commercial widgets, with old (jQuery UI) widgets open-sourced on GitHub (their development stopped in 2013). Developed by ComponentOne, a division of GrapeCity. See Wijmo Complete vs. Open.
CxJS - commercial JS framework based on React, Babel and webpack offering form elements, form validation, advanced grid control, navigational elements, tooltips, overlays, charts, routing, layout support, themes, culture dependent formatting and more.

CxJS

Widgets - Demo Apps - Examples - GitHub

Full-stack frameworks

SproutCore - developed by Apple for web applications with native performance, handling large data sets on the client. Powers iCloud.com. Not intended for widgets.
Wakanda: aimed at business/enterprise web apps - see What is Wakanda?. Architecture:
- Wakanda Server (server-side JavaScript (custom engine) + open-source NoSQL database)
- desktop IDE and WYSIWYG editor for tables, forms, reports
- Wakanda Application Framework (datasource layer + browser-based interface widgets) that helps with browser and device compatibility across desktop and mobile
  
  Wakanda is highly integrated, includes a ton of features out of the box, but has a very small GitHub community and SO presence.
Servoy - "a cross platform frontend development and deployment environment for SQL databases". Boasts a "full WYSIWIG (What You See Is What You Get) UI designer for HTML5 with built-in data-binding to back-end services", responsive design, support for HTML6 Web Components, Websockets and mobile platforms. Written in Java and generates JavaScript code using various JavaBeans.
SmartClient/SmartGWT - mobile and cross-browser HTML5 UI components combined with a Java server. Aimed at building powerful business apps - see demos.
Vaadin - full-stack Java/GWT + JavaScript/HTML3 web app framework
Backbase - portal software
Shiny - front-end library on top R, with visualization, layout and control widgets
ZKOSS: Java+jQuery+Bootstrap framework for building enterprise web and mobile apps.

CSS libraries + minimal widgets

These libraries don't implement complex widgets such as tables with sorting/filtering, autocompletes, or trees.

Bootstrap
Foundation for Apps - responsive front-end framework on top of AngularJS; more of a grid/layout/navigation library
UI Kit - similar to Bootstrap, with fewer widgets, but with official off-canvas.

Libraries using HTML Canvas

Using the canvas elements allows for complete control over the UI, and great cross-browser compatibility, but comes at the cost of missing native browser functionality, e.g. page search via Ctrl/Cmd+F.

Zebra - demos

No longer developed as of Dec 2014

Yahoo! User Interface - YUI, launched in 2005, but no longer maintained by the core contributors - see the announcement, which highlights reasons why large UI widget libraries are perceived as walled gardens that developers don't want to be locked into.
echo3, GitHub. Supports writing either server-side Java applications that don't require developer knowledge of HTML, HTTP, or JavaScript, or client-side JavaScript-based applications do not require a server, but can communicate with one via AJAX. Last update: July 2013.
ampleSDK
Simpler widgets livepipe.net
JxLib
rialto
Simple UI kit
Prototype-ui

Other lists

Best of JS - component toolkits
Wikipedia's Comparison of JavaScript frameworks
Wikipedia's list of GUI-related JavaScript libraries
jqueryuiwidgets.com - detailed jQuery widgets feature comparison