When first starting out as more of a hobbyist one of the first things I struggled with was how best to use objects and the object structure to store and use data. When to use OLOO vs. a class-like structure to properly scope functions, when to use prototypes, etc. Much of this has been "figured out" or at least made easier in the latest versions of ECMA which is nice.
Working with promises / async and chaining multiple things together to ensure state came together properly (in the jQuery days) and you didn't get "undefined" for your variable that logged just fine in the console was the next annoyance. Abstractions for these are pretty high quality now and somewhat easier to use but I definitely recommend gaining understanding before using otherwise it will bite you eventually :)
this (as has been said) is also an issue for me. Can still bite but at least I know to think "hey which this is this?" now.
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