Yes, that is fine. By default (IIRC) replit/node.js will call index.js, if you want it to call something else, you will need to modify the .replit file.
You can use your index.js as a file for your functions. You can rename it to functions.js, and add your functions like module.exports = { YOUR, FUNCTIONS, HERE ); at the bottom of your file.
Then, rename tester.js to index.js and then imports the functions by adding require('./functions'); at the top of your file and assigning it to a variable.
From there, you can use the functions like how you would use them as if they are already defined like this: