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
Quelly
Wait, the return; in your callback is what makes the odd slots come out undefined, but when you do out. join(', ') aren’t those undefined entries coerced into empty strings (so you’d see ,4, ), or am I misremembering how join treats undefined? I might be wrong here.
You’re remembering right: join turns undefined (and null) into '', so you get extra separators with “nothing” between them. The only time you don’t see it is with actual holes in a sparse array — join treats those the same way (empty), which is why it’s easy to confuse.
Look — map gives you a dense array, so when your callback returns nothing you get actual undefined values, and join turns those into '' but still prints the separators.
Holes are different: a bunch of methods will skip them entirely, but join treats holes the same as undefined/null, which is why the output is so easy to misread.
