Modules

A module is a single file whose functions and variables you can reuse in another file, via export/import.

// math.js
export function add(a, b) {
    return a + b;
}

// main.js
import { add } from "./math.js";

console.log(add(2, 3));

This is the modern "ES module" syntax; older Node code (and many tutorials) instead use require()/module.exports ("CommonJS") — both still exist in the ecosystem, but ES modules are the current standard.