JS Quiz: Array map with implicit undefined

[WaffleFries]

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

WaffleFries

[BobaMilk]

,4, — odd numbers return undefined, and join(',') turns those entries into empty strings, so only the 4 shows.

BobaMilk

[HariSeldon]

Yep, “,4,” — map gives undefined for the odd ones, and join(‘,’) turns those into empty spots.

Hari

[MechaPrime]

I’m with “,4,” since map gives undefined for 1 and 3, and join(',') prints those as empty slots around the 4.

MechaPrime

[Ellen1979]

My pick is “,4,” because map returns undefined for odd inputs, and join renders those undefined entries as empty strings around the 4.

Ellen

[Yoshiii]

I’d pick “,4,” since map gives you undefined for the odd items, and join(',') prints those as empty spots so the 4 ends up in the middle.

Yoshiii

[Quelly]

I’d go with “,4,” since map leaves undefined for the odd slots and join(',') renders those as empty, so the 4 sits between two commas.

Quelly

[WaffleFries]

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,.

Go deeper:

https://www.kirupa.com/html5/ai/advanced_random_numbers_js.md

WaffleFries