[jasonkillian]

Wouldn't recommend doing this - if the original array is of significant length this'll get quite slow because `acc.concat` has to create a brand new array of slightly longer length on each iteration it's called. Better to just use `push` like you suggested before and then return the array if you want to use `reduce`.

[ReleaseCandidat]

Yes, of course, that's why I used `push` at first.

[jacobolus]

Use the comma operator: (acc.push(2*n), acc) will return acc. Or e.g.

  [1, 2, 3, 4, 5].reduce((acc, n) => (n % 2 ? acc.push(2*n) : null, acc), [])

[wk_end]

If you're just iterating through the array and mutating an object on each iteration, just use a for loop.

[jacobolus]

Obviously you can alternately write:

  let input = [1, 2, 3, 4, 5], output = [];
  for (let i = 0; i < input.length; ++i) {
    let n = input[i];
    if (n % 2) output.push(2*n);
  }
  return output;

But in some circumstances the other style can be more convenient / legible. The immediate question was about pushing to an array and then returning the array, for which the comma operator can be handy.

[wk_end]

No argument that the comma operator is a neat trick when you need it.

FWIW, it's 2022:

  const output = [];
  for (const n of [1, 2, 3, 4, 5]) {
    if (n % 2) output.push(2 * n);
  }

[eyelidlessness]

Minority opinion: please `let` your mutable references. I know `const` doesn’t signal immutability, but we as humans with eyeballs and limited attention span certainly benefit from knowing when a value might change at runtime.