01

Create a new file in the same directory, with function to export as an object with module.exports this example contains math functions in math.js

const add = (a,b) => a + b
const subtract = (a,b) =>  a - b
const multiply = (a,b) =>  a * b
const divide = (a,b) => a / b
module.exports = {add,substact,multiply,divide}

02

Import them into the server with require('./')

you must use specific path when not using Common Core Modules. You do not need .js

const math = require('./math')
console.log(math.add(4,7))
console.log(math.multiply(3,5))