For me the extremely weak type system is one thing. In general the language is too accepting of bad code. It's kinda like HTML, "if you can guess at what it's supposed to do, do it." Personally, I prefer stricter interpretation over weaker versions.
Another big one for me is "this", which has significantly different semantics than nearly any other OO language out there. I personally find it confusing, and it requires you in many cases to understand the invocation point of a function. I can not reason about all the data related to the scope of the function, in the function, without first taking into consideration all the callers of the function.
To be fair though, JS is not the only language that features these flaws. The biggest problem with it is that I'm not really given a choice to use other languages. Yes, there are transpilers, but even with those I almost always need to use JS at some point for integration with other code.
Another big one for me is "this", which has significantly different semantics than nearly any other OO language out there. I personally find it confusing, and it requires you in many cases to understand the invocation point of a function. I can not reason about all the data related to the scope of the function, in the function, without first taking into consideration all the callers of the function.
To be fair though, JS is not the only language that features these flaws. The biggest problem with it is that I'm not really given a choice to use other languages. Yes, there are transpilers, but even with those I almost always need to use JS at some point for integration with other code.