CommonJS Modules

module.exports

Each file is its own module. Expose values with module.exports.

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

module.exports = { add };

require()

Pull a module in with require — Node caches it after the first load.

// app.js
const { add } = require("./math");

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

Output:

5

Synchronous by nature

require loads modules synchronously — fine for local files, but that's one reason ES modules were introduced.