`module.exports` in Node.js

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 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(). This gives developers control over which parts of a file can be accessed externally, allowing for encapsulation.

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

// 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().

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