# Node.js module.exports: CommonJS Module System

> Use module.exports in Node.js to define which functions and variables a CommonJS module exposes when imported with require(), enabling encapsulation

**URL:** https://sentry.io/answers/module-exports-in-node-js/

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## The problem

What is the purpose of Node.js's `module.exports` and do I use it?

## The solution

Node.js uses the [CommonJS module system](https://en.wikipedia.org/wiki/CommonJS) to allow developers to distribute their code across multiple files and make use of third-party libraries. `module.exports` is part of the CommonJS specification – it defines the object that is created when a file is imported using [`require()`](https://nodejs.org/api/modules.html#requireid). This gives developers control over which parts of a file can be accessed externally, allowing for [encapsulation](https://en.wikipedia.org/wiki/Encapsulation_(computer_programming)).

Consider the following module code, which provides methods for retrieving names in different formats.

```javascript
// names.js module
function lastname() { // private function
  return "Doe";
}

function firstname() { // private function
  return "Jane";
}

function firstnameLastname() { // public function
  return `${firstname()} ${lastname()}`;
}

function lastnameFirstname() { // public function
  return `${lastname()}, ${firstname()}`;
}

module.exports = { firstnameLastname, lastnameFirstname }; // object to be returned by require()
```

This module contains four functions, but only the last two are included in `module.exports`. If we `require()` this module in another file, we will not be able to call `firstname()` or `lastname()`.

```javascript
const names = require("names.js"); // require() returns the object assigned to module.exports

console.log(names.firstnameLastname()); // will print "Jane Doe"
console.log(names.lastnameFirstname()); // will print "Doe, Jane"

console.log(names.firstname()); // will produce an error
console.log(names.lastname()); // will produce an error
```

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*Source: [sentry.io/answers/module-exports-in-node-js/](https://sentry.io/answers/module-exports-in-node-js/)*