Javascript 如何匹配回调函数中的参数？

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 example in the following forEach loop:

 var friends = ['Mike', 'Stacy', 'Andy', 'Rick']; friends.forEach(function(eachName, index){ console.log(index + 1 + ". " + eachName); });

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?

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

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.

Answer 2

Ya it's by default.To start using a function better if you refer some API.

I have used MDN for that.