How does Javascript match the parameters in the callback function? -

i started learn javascript, , callback functions seem hard understand. 1 question have how javascript matches parameters in callback function? example in following foreach loop:

var friends = ['mike', 'stacy', 'andy', 'rick'];     friends.foreach(function(eachname, index){       console.log(index + 1 + ". " + eachname);     }); 

is default foreach function pass index second parameter , entry first parameter in callback function?

in order master callback functions, need check api (in case, foreach) every time use it?

is default foreach function pass index second parameter , entry first parameter in callback function?

yes; this part of specification. in fact, passes array being iterated third argument.

call [[call]] internal method of callbackfn t value and argument list containing [the value], [the index], , [the object].

(emphasis mine.)

in order master callback functions, need check api (in case, foreach) every time use it?

well, they’re pretty consistent each other, you’ll remember @ point. map, filter, every, , some work in way.