this is good

In theory, the complexity of this is worse than the optimal O(n). However... since this is JavaScript and it can only be correct to up to 15 digits... I guess it's practically acceptable.

how does this work? I don't understand what the function is returning.

okey thank you

This does not scale well. Have you tried any n greater than 1,000,000?
Besides, your for loop should start with int i = 3 obviously because this is the first divisible number by 3.

Indeed, 33, 35, and 42 are struck-through, so they are not being taken into account.

There's, clearly, a mistake in the example. Considering eligible numbers (divisible by 3 or 7) must not exceed N (30 in your example), 33, 35, 42 should be excluded. Furthermore, because of that the actual sum of #1, #2, #3 must be greater, which is most obvious for #3.

I still don't understand how [[a]] extracts the first character without using spread operator o_O

ok, thanks for pointing that out

Nice!

done

Let's golf :D

This one will drop off words beyond 3-rd (e.g. double-barrelled surname divided by space instead of hyphen), so 3 extra dots seems to be a reasonable cost.

Use spoiler flag next time please.