As first framework to learn. I think I should be React.
And how you will understand you already learned React enough? You need adequate metrics.
Learning React is about have good understanding of state-setState, and component life cycles.
Write authentication with React, then same with Redux, and then try mobx.
Adopt flow-type for you applications.
Maybe then it will be interesting to try different solutions, for example Angular4 or Vue.
You only at the beginning
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