ExtJS is a quite powerful class library. It has it's own class and inheritance System, which you'll have to get familiar with. There are also Integrations for React and Angular.
there's so much functionality in the framework, you can Do almost anything by setting a simple config.
Since Sencha got acquired by Idera, Support quality seems to suck and there's a lot of doubt regarding the future. But for now I'd say, that it's a good Tool.
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