I just recently implemented my own parser combinator lib in typescript too. It was surprisingly simple in the end.
This function parses a number in 6502 asm. So `255` in dec or `$ff` in hex: https://github.com/geon/dumbasm/blob/main/src/parsers/parseN...
I looked at several typescript libraries but they all felt off. Writing my own at least ensured I know how it works.