I want to ask if there are any JavaScript methods you need to know, such as the slice method for arrays (this is an example). I ask this question because I have difficulty implementing the methods and using them correctly.
Definitely try to get the hang of .reduce() --it’s an amazingly powerful method. Start with implementing Math.max with it, then try your hand at implementing .map() and .filter().
Strings: padStart/padEnd for formatting numeric strings.
Dates: toLocaleString for formatting dates (it is extremely flexible).
Browser API: async/await
Destructuring for grabbing object properties. Cannot emphasise enough how useful this is once it becomes muscle memory and you just automatically use it.
map, filter, includes, every, join for arrays.
It is imo important to understand reduce well, because almost every other array method can be implemented using it. I used to use it a lot for changing the shape of data in lists, but it tends to be difficult to [easily] follow the code once you get past very simple usecases. So instead of reduce, or cases where I need to [say] filter then map then do something else generally use:
for...of loops.
Rest/spread, in particular [...someArray] for copying arrays and {...someObject} for copying/non-destructively updating objects.