find some simple project you could do in a week; an RSS reader is my goto
pick a frontend + a backend technology you're reasonably comfortable with (React + Node.js might be a good fit for you)
build it and see which side you like more
Something else to consider is that even if you want to focus on backend, you still have some frontend experience. It won't suddenly disappear. Find opportunities to do both; it'll only make you more valuable.
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Something else to consider is that even if you want to focus on backend, you still have some frontend experience. It won't suddenly disappear. Find opportunities to do both; it'll only make you more valuable.