How does Javascript match the parameters in the callback function?

Question

I just started to learn Javascript, and callback functions seem hard to understand. One question I have is how javascript matches the parameters in a callback function? for exam

Answer 1

Is it by default that the forEach function will pass index to the second parameter and entry to the first parameter in the callback function?

Yes; this is part of the specification. In fact, it also passes the array being iterated as the third argument.

Call the [[Call]] internal method of callbackfn with T as the this value and argument list containing [the value], [the index], and [the object].

(Emphasis mine.)

In order to master callback functions, do I need to check the API (in this case, forEach) every time I use it?

Well, they’re pretty consistent with each other, so you’ll remember at some point. map, filter, every, and some also work in this way.