You can just return an object without parenthesis ()

It works until there is NaN or Inf in ls or there is loss of precision.

😄

thanks a lot! It doesn't allow me switch the flag now :(

That's dangerous. If you skip it and the array has a length of one, you'll try to add the first element with undefined. So it's better practices to add it anyway (only case where you can forget it is when you're absolutely sure there are at least 2 elements.

What you cannot skip, tho: the spoiler flag. ;p

Using map for side effects is generally a yikes, but I think here it's kind of beautiful in a golfy way

The only bad thing about this solution is that it modifies the input. Still very clever and elegant.

Honestly, if you can't read this, I couldn't explain.

If you don't know where to look up &, try here.

how does it work?

Even though this solution traverses the entire array twice, I still think it is clever because it is so idiomatic. Nice job.

Why not assign i = 1 and not add 1 to i each loop? Is there a reason?