First try to bf the result for 10/100/1000/10000/100000. Then analyse each and every result and find out the connection between them. This is the way i've solve it but it took me 2-3 hours. No higher math or 'magic' formula is needed. Good Luck.

you can log the input to the console using the print function

It's an array of a single character which holds the amount of vowels.

For some reason.

God knows why they didn't just give a variable but you know, it doesn't matter.

you're probably printing the answer instead of returning it

Actual: "() - " Expected: "(123) 456-7890"

means that your function returned "() - " when the answer is "(123) 456-7890"

nb_vowels is the number of vowels. You need to report the number of vowels into it.