You can reference things from separate modules:

index.js:

import { say } from "./lib1.js";

say("Hello, world!");

lib1.js:

export function say(message) {
    console.log("The user said: " + message);
}

If you just want to write separate modules:

index.js:

import "./lib1.js";

lib1.js:

console.log("This script will run because index.js references it");