I think I have to agree with this article!
It can be very tempting to jump from one bandwagon/fad/language to the next, and you will be able to write "hello world" in 30+ languages. But getting really good at one language/framework will make you a better developer in the end: diving into the internals of the framework's source code, contributing to the ecosystem, solving "hard" problems, and so on.
I've also come to believe that choosing a "niche" wil actually make it easier to market yourself (for instance as a freelance dev), because it's immediately clear to a potential client what your expertise is, rather than the somewhat overused "full stack web dev" moniker.
Having said that, knowledge of a broad range of topics is also essential, because it gives you the context and background to be able to come up with solutions. So it's a bit of "and ... and", not "either": have general knowledge about a broad range of topics, and go deep in a few.
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