This is very slow for certain values (i. e. 1900000000).

Use spoiler flag next time please.

First argument is the pattern we want to find in the string. The pattern can be a string or Regex. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp

Second argument is the replacement. It can be a string or a function. If the string is emtpty, we essentially just remove the pattern.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace

What does .replace() do?

same solution)

It's regular expressions, they are not supposed to be readable. It's a great tool that you shouldn't avoid just because it doesn't look pretty.

Inefficient, but super clever!

Good grief. Mine is awfully clumsy by comparison... Very nice.

I dont understand this solution. Can you explain it to me sir? I would like to learn that

I really like this solution. My one thought is if there are potentially a lot of duplicates, would it make sense to check if words[n] exists before setting calling num2word again?

While it's true that this could be approximately twice as fast, the complexity is still the same (constant factors are ignored).

yeah, great solution.