what use does the javascript forEach method have (that map can't do)?

Question

The only difference I see in map and foreach is that map is returning an array and forEach is not. However, I don\'t even understand the last line

Answer 1

This is a beautiful question with an unexpected answer.

The following is based on the official description of Array.prototype.map().

There is nothing that forEach() can do that map() cannot. That is, map() is a strict super-set of forEach().

Although map() is usually used to create a new array, it may also be used to change the current array. The following example illustrates this:

var a = [0, 1, 2, 3, 4], mapped = null;
mapped = a.map(function (x) { a[x] = x*x*x; return x*x; });
console.log(mapped); // logs [0, 1, 4, 9, 16]  As expected, these are squares.
console.log(a); // logs [0, 1, 8, 27, 64] These are cubes of the original array!!

In the above example, a was conveniently set such that a[i] === i for i < a.length. Even so, it demonstrates the power of map(), and in particular its ability to change the array on which it is called.

Note1:
The official description implies that map() may even change length the array on which it is called! However, I cannot see (a good) reason to do this.

Note2:
While map() map is a super-set of forEach(), forEach() should still be used where one desires the change a given array. This makes your intentions clear.

Hope this helped.