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Pro Easy to reuse components
Pro Virtual DOM support
Instead of relying on the DOM, React implements a virtual DOM from scratch, allowing it to calculate precisely what needs to be patched during the next screen refresh. This is orders of magnitude faster than fiddling with the DOM itself.
Pro One-way data flow
React's one-way data binding (or one-way data flow) means that it's easy to see where and how your UI is updated and where you need to make changes. It's also very easy to keep everything modular, fast and well-organized.
Pro Supported by Facebook and Instagram
React is built by Facebook engineers initially to be used only for their inner projects especially to solve the problem of building large complex applications with constantly changing data.
Pro Can be used with different libraries
Pro Template engine independent
React provides a template engine (JSX) which is easy to use. But it's not mandatory.
Pro Server side rendering
React can render it's components and data server side, then it sends those components as HTML to the browser.
This ensures faster initial loading time and SEO friendliness out of the box, since it's indexed as any other static website by search engines.
Pro Functional programming style leads to less buggy UIs
Pro Easy to write tests
Pro Good debugging tools
React has an official Chrome Extension which is used as a developing and debugging tool. It can be used to quickly and painlessly debug your application or view the whole application structure as it's rendered.
Pro Flux architecture pattern
Flux is a platform agnostic pattern which can technically be used with any application or programming language.
One of Flux' main features is that it enforces uni-directional data flow which means that views do not change the data directly.
With React this is useful because this way it's easier to understand an application as it starts getting more complicated. By having two-way data binding, lead to unpredictable changes, where changing one model's data would end up updating another model. By using the Flux architecture, this can be avoided.
Pro Extensive SVG support
Since React v0.15, SVG is fully supported. React supports all SVG attributes that are recognized by today's browsers.
Pro Keep control over your app's logic
React is just a view library, so you still have (almost) full control over how your app behaves.
Pro Supported by ClojureScript libraries
Reagent, Om, Rum, etc.
Pro Tested on Facebook itself
React is used on one of the most visited websites on the planet, Facebook. With stellar results and with millions of people experiencing it every day.
Pro Comprehensive documentation
The Sencha documentation is comprehensive, with detailed documentation and a number of examples displaying the various widgets, tools and themes.
Pro Supports MVC and MVVM development
Pro Supports Web and Mobile deployment out of the one framework or codebase
Pro Support for easy theming of applications
Pro Visual Design tool available
The Sencha Architect product allows you to visually build your application, or rapidly prototype a system.
Pro IDE Plugins available
A number of plugins are available for some of the commonly used IDEs (eg: JetBrains, Eclipse, Visual Studio), providing templates, refactoring support, hinting and code completion/generation, as well as management of includes and other time-saving features.
Pro Charting package included
Con Heavy on memory
React's virtual DOM is fast, but it requires storing elements in the virtual and real DOM increasing memory usage for the page. This can be a real problem for single-page webapps designed to be left running in the background.
Con Not a complete solution
React does not do everything for the developer, it's merely a tool for building the UI of a web app. It does not have support for routing or models, at least not out of the box. While some missing features can be added through libraries, to start using React and use it in production, you still would need to have experience, or at least a good grasp on what the best libraries to use would be.
Con Large file size
react.min.js is 145.5KB in size. It's much larger than some other libraries that offer roughly the same features and it's almost the same size as some MV* frameworks such as Angular or Ember that offer more features out of the box.
Although, it should be mentioned that sometimes having a smaller library may force developers to reinvent the wheel and write inefficient implementations on features that React already has. Ending up with a larger application that's harder to maintain and/or that has bad performance.
Con Renders too frequently
Con You have to learn a new syntax
Requires learning a custom syntax, JSX, that has some gotchas and introduce complexity, a steeper learning curve, and incompatibility with other tools.
Though you can opt out from JSX and use vanilla JS instead. But that is not recommended since it adds a lot of unneeded complexity which JSX tries to avoid.
Con Template(view) mixed into code
Con No support for legacy browsers
React has recently dropped support for Internet Explorer 8. While the library may still work on IE8, issues that affect only IE8 will not be prioritized and/or solved.
Con Sencha CMD is bloated and frustrating to work with
To do any meaningful development, you are stuck with CMD. There is a gulp task that will handle the JS concatenation, but there is nothing outside of CMD that can handle theming in their ecosystem.
In addition, CMD is based on Java, and is very heavy to run (600MB+ on Windows 10 to watch for changes in the application and recompile).
Con Sencha CMD (their CLI) is under documented, and out of date
Their latest release of CMD changed some configuration locations, but the documentation was not updated to reflect this. There is no reference guide on the json configuration files, other than the (unfortunate use of) comments in the generated json files.
Con They use proprietary extensions to SASS, making it incompatible with anything but their Fashion processor
On the plus side, you do not have to install ruby alongside CMD for more recent versions of ExtJS. However, their Fashion processor seems to only be available through CMD.
Con Too often breaking changes between versions. They have little concept of backwards compatibility
Compounded by the fact that there are now two "toolkits" in the same "version" of ExtJS, with certain components not existing in one vs the other.
Con The IDE tools are not sold separately - you must purchase the appropriate license pack
You get all the IDE plugins, even if you only need one. They should offer sell them individually, or continue to bundle them with a dev license pack.
Con Difficult to integrate with 3rd party software
Any third party library you wish to include has to be wrapped in some sort of component adapter. You have to do a lot of tweaking to get the build process right if you want the 3rd party lib to be bundled into your application in the right order.
Con Can be expensive
The framework is a commercial package, and the recent decision to start with a minimum of 5 users may rule out smaller developer teams or startups. Recently, they have started a program that allow essentially what are contractors to purchase single licenses, but not individual, independent developers.