What's the difference between map and forEach?

Quick answer

map returns a new transformed array; forEach returns undefined and is used for side effects.

In detail

`map` builds a new array of the same length from each item's return value, so it's chainable and non-mutating. `forEach` just runs a callback per item, ignoring the return value, and can't be chained or broken out of. Use map to transform, forEach for side effects.

[1, 2, 3].map(n => n * 2);     // [2,4,6]
[1, 2, 3].forEach(n => save(n)); // undefined

Why interviewers ask this: Basic but reveals whether you think in transformations.

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