The one-liner is using built-in methods to do the conversion ([0, 0, 0, 1] -> ["0", "0", "0", "1"] --> "0001" --> 1)

Whilst yours is doing the mathematical approach.

The code for string replacement is wrong because you intended to replace all substrings with positioning beyond counter whilst the correct way is to replace the specified character with respective position.

🤣

I read somewhere that the return command exits the whole function. This means that the loop wouldn't be repeated since it is no longer running the function where the loop is in.

Edit: fixed some grammar and punctuation errors

lmao what is a 25 dollar bill

I got it! Thank you. I had the else block on the wrong indent, I should have inlcuded it with the "x == 100" part. Thanks again!

Look closely what happens when they pay you with a 100 dollars bill. Read this too: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution