JavaScript: Difference between .forEach() and .map()

Question

I know that there were a lot of topics like this. And I know the basics: .forEach() operates on original array and .map() on the new one.

In my case:

function practice (i){
    return i+1;
};

var a = [ -1, 0, 1, 2, 3, 4, 5 ];
var b = [ 0 ];
var c = [ 0 ];
console.log(a);
b = a.forEach(practice);
console.log("=====");
console.log(a);
console.log(b);
c = a.map(practice);
console.log("=====");
console.log(a);
console.log(c);

And this is output:

[ -1, 0, 1, 2, 3, 4, 5 ]
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
undefined
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
[ 0, 1, 2, 3, 4, 5, 6 ]

I can't understand why using practice changes value of b to undefined.
I'm sorry if this is silly question, but I'm quite new in this language and answers I found so far didn't satisfy me.

Answer 1

They are not one and the same. Let me explain the difference.

forEach: This iterates over a list and applies some operation with side effects to each list member (example: saving every list item to the database)

map: This iterates over a list, transforms each member of that list, and returns another list of the same size with the transformed members (example: transforming list of strings to uppercase). It does not mutate the array on which it is called (although if passed a callback function, it may do so).

References

Array.prototype.forEach() - JavaScript | MDN

Array.prototype.map() - JavaScript | MDN