Node.js CommonJS Modules

What you will practice

• Understand why modules exist in Node.js.
• Recognize CommonJS terms.
• Practice separating reusable functions.

What this means

Node.js has historically used CommonJS modules with `require` and `module.exports`.

`module.exports` chooses what a file shares. `require` loads what another file exported.

Modern Node.js can also use ES modules with `import` and `export`, but many real projects still contain CommonJS.

Mini glossary

require

CommonJS syntax for loading another module.

module.exports

The value a CommonJS file shares with other files.

Module boundary

The line between private code inside a file and public code exported from it.

How it works step by step

1. Create `math.js` and export functions with `module.exports`.
2. Create `index.js` and load the exported functions with `require`.
3. Run `node index.js` from the same folder.
4. Change one function in `math.js` and rerun the app to see how modules keep logic reusable.

Where you will use this

• A Node.js app exports database helpers from one file.
• A test file requires a function and checks its behavior.
• A CLI project splits parsing, validation, and output into modules.

Before you run the code

The syntax is different from ES modules, but the idea is the same: share focused code across files.

As a beginner, care more about the boundary than the exact module flavor. Later you can compare CommonJS with ES modules.

Run it locally

These examples are meant for a local Node.js project. Create the files shown below, then run the terminal commands from the project folder.

mkdir lumio-commonjs-demo
cd lumio-commonjs-demo
touch math.js index.js

function add(a, b) {
  return a + b;
}

function average(numbers) {
  const total = numbers.reduce((sum, value) => sum + value, 0);
  return total / numbers.length;
}

module.exports = {
  add,
  average
};

const { add, average } = require("./math");

const scores = [8, 10, 9];

console.log(`Total: ${add(4, 6)}`);
console.log(`Average score: ${average(scores)}`);

node index.js

Common beginner mistakes

• Exporting too much and making every file depend on every other file.
• Confusing CommonJS with ES modules.
• Copying functions between files instead of exporting reusable logic.

Try changing this next

• Add a `triple` function to the exported object.
• Rename `mathFile` to `calculatorFile`.
• Print the names of all exported functions.