[weatherlight]

that's not equivalent.

      d.map(i => o[i] = i\*2)

Will produce a new array on top of mutated data, o, in the parent scope.

      d.forEach(i => o[i] = i**2)

would be the one-to-one equivlent

[peterthehacker]

Both mutate o. If you ignore the array returned by map then they’re equivalent.