Array.map 1 element to multiple element

Question

I have [3, 16, 120]. when I do [3, 16, 120].map(mapper), I want to achieve, for example [4,5, 17,18, 121,122] i.e. each element map to n

Answer 1

Immutable solution, with the spread operator:

[3, 16, 120].reduce((a, v) => [...a, v+1, v+2], [])