Mine is that the useState hook is an actual plague on the entire React ecosystem. Regardless of your opinions of React, if you wanna use something then use it right.
No single letter variable names.
Yes I also mean the i in for loops you see in tutorials. Write out "index" or give it a more descriptive name. Doing the typing isn't the hard part of programming, nor the most time consuming, so making it take a teeny tiny bit longer by having longer variable names is more than worth the value of variables the effortlessly describe themselves.
line length is not that big of a problem anymore, we're not on CRTs, people are regularly developing with 4k monitors, we don't need to limit to 40 character lines but even still if you can't possibly think of a way to break up that long of a line and the only thing you can think of to make it shorter is using over-acronymized variable names then you've already lost the plot somewhere