Once you work with several languages it’s really hard to keep track of all this stuff. I never know what exactly evaluates to true vs false in PHP or JavaScript for example. Add to that sloppy programmers on your team who don't check their assumptions it’s really easy to have a ton of subtle bugs in your code.

> not even knowing how JavaScript implements map in a nonstandard way that breaks simple examples

FTFY.

I know exactly how map works: https://en.m.wikipedia.org/wiki/Map_(higher-order_function)

This is map:

> a higher-order function that applies a given function to each element of a functor, e.g. a list, returning a list of results in the same order.

Since that is not what Array.prototype.map does, it is not map, it is some similar thing that is misnamed as map. If I wanted the index and the array as arguments, I would ask for them.

I would certainly consider this a "gotcha" because it's unexpected that map both deviates from the norm and the language makes it so easy to misuse and hide that misuse.

This reminds me of "array set" in Tcl (a language many probably haven't used in a long time -- or ever). Unlike the normal "set" command in Tcl, which overwrites the full value of the variable, "array set" is actually a merge operation -- it merges in new keys and doesn't get rid of anything. I've seen experienced programmers use "array set" in a loop, leading to awful bugs. This too could be dismissed by saying that they "don't even know how array set works," but actually I place the blame on the language for making it so easy to misuse.