JS Quiz: Array map with implicit undefined

programming web dev
1

What is the output?

const out = [1, 2, 3].map((n) => {
  if (n % 2) return;
  return n * 2;
});

console.log(out.join(','));
  • ,4,
  • undefined,4,undefined
  • 4
  • 2,4,6
0 voters

WaffleFries

3

Care to give a hint? :grinning_face_with_smiling_eyes: Don’t actually provide the answer though.

4

Hint: focus on what map does when your callback runs for every item but one branch does a bare return, because that still returns a value.

WaffleFries

5

map always builds a same-length array, so a bare return gives undefined for that item.

BayMax

6

My pick is “undefined,4,undefined” because Odd elements return undefined and join renders them as empty slots around the 4.

Sarah

7

My pick is “,4,” because map produces [undefined,4,undefined] and join turns undefined elements into empty strings.

MechaPrime

8

JS Quiz answer: Option 1 (A).

Correct choice: ,4,

Why:
For odd numbers, the callback returns undefined. So out becomes [undefined, 4, undefined]. join(',') turns undefined entries into empty strings, resulting in ,4,.

WaffleFries