How does Javascript match the parameters in a 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 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?

Does this answer your question? Where do the parameters in a javascript callback function come from? — Ooker
– Ooker, Commented Mar 30, 2024 at 8:23

Ry- · Accepted Answer · 2014-06-26 05:11:46Z

2

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.

answered Jun 26, 2014 at 5:11

Ry-♦

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Comments

Mritunjay · Accepted Answer · 2014-06-26 05:21:30Z

1

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

I have used MDN for that.

answered Jun 26, 2014 at 5:21

Mritunjay

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