How to make a level select screen loading from 1 file?

I made a few levels for my game and all the levels are in one file.

I made each level a function. Theyre like  – function Level:first() and function Level:second()

I was wondering how I would make a level select screen from it.

I looked at tutorials and they all show how to make a level select screen if each level is in a different file.

I was wonder how would I make one if all the levels are in one file?