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

While all the previous questions are correct, I would definitely make a different distinction. The use of map and forEach can imply intent.

I like to use map when I am simply transforming the existing data in some way (but want to make sure the original data is unchanged.

I like to use forEach when I am modifying the collection in place.

For instance,

var b = [{ val: 1 }, { val: 2 }, { val: 3 }];
var c = b.map(function(el) {
    return { val: el.val + 1 }; // modify element in-place
});
console.log(b);
//  [{ val: 1 }, { val: 2 }, { val: 3 }]
console.log(c);
//  [{ val: 3 }, { val: 4 }, { val: 5 }]

My rule of thumb being making sure when you map you are always creating some new object/value to return for each element of the source list and returning it rather than just performing some operation on each element.

Unless you have any real need to modify the existing list, it doesn't really make sense to modify it in place, and fits better into functional/immutable programming styles.