Node.js Modules and require Interview Questions CommonJS vs ES Modules

Node.js Modules and require Interview Questions

The module system is a core Node.js concept. Here are the most common interview questions.

Q1: What Is the Difference Between require and import?

Answer:

• require (CommonJS) Synchronous, used in Node.js traditionally
• import (ES Modules) Asynchronous, standard JavaScript, supported in Node.js 13+

// CommonJS
const express = require("express");
module.exports = { myFunction };

// ES Modules
import express from "express";
export { myFunction };
// or
export default { myFunction };

Q2: How Does require() Work?

Answer: require() follows these steps:

1. Resolution Find the file (core module, node_modules, or file path)
2. Loading Read the file content
3. Wrapping Wrap in a function: (function(exports, require, module, __filename, __dirname) { ... })
4. Evaluation Execute the wrapped function
5. Caching Cache the module for future requires

Q3: What Is Module Caching?

Answer: Node.js caches every required module. The second require returns the cached module:

// counter.js
let count = 0;
module.exports = { increment: () => count++, getCount: () => count };

// app.js
const counter1 = require("./counter");
const counter2 = require("./counter"); // Same object as counter1
counter1.increment();
console.log(counter2.getCount()); // 1   same instance

To clear cache: delete require.cache[require.resolve("./counter")]

Q4: What Are Circular Dependencies and How Does Node.js Handle Them?

Answer: Circular dependencies occur when module A requires module B, and B requires A.

// a.js
const b = require("./b");
console.log("a.js loaded, b =", b);

// b.js
const a = require("./a");
console.log("b.js loaded, a =", a);

Node.js handles this by returning a partial (incomplete) module.exports object. When b.js requires a.js, it gets the current state of a's exports (which may be incomplete).

Best practice: Avoid circular dependencies by refactoring shared logic into a third module.

Q5: What Is the Difference Between module.exports and exports?

Answer:

// These are the same initially:
exports === module.exports // true

// WRONG   reassigns exports, breaks the reference
exports = { myFunc: () => {} };

// RIGHT   adds to module.exports
exports.myFunc = () => {};
// or
module.exports = { myFunc: () => {} };

Always use module.exports = ... to be safe.

Q6: What Are Core Modules in Node.js?

Answer: Core modules are built into Node.js:

• http / https HTTP server/client
• fs File system
• path Path utilities
• os OS information
• crypto Cryptography
• events Event emitter
• stream Streams
• util Utilities
• child_process Spawn processes
• url URL parsing

const fs = require("fs"); // Core module
const express = require("express"); // node_modules
const myModule = require("./myModule"); // Local file

The Takeaway

Module system interview questions cover: require vs import (CommonJS vs ES Modules), how require works (resolve, load, wrap, evaluate, cache), module caching (same instance on repeated requires), circular dependencies (partial exports avoid by refactoring), module.exports vs exports (always use module.exports), and core modules (http, fs, path, crypto, events, stream).