Modules — Tricky Interview Questions

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Q1: exports vs module.exports

javascript// module.js
exports.a = 1;
exports = { b: 2 }; // reassignment
exports.c = 3;

// What does require('./module') return?

Answer: { a: 1 }

Why: exports.a = 1 adds to the shared object (fine). exports = { b: 2 } breaks the reference — exports now points to a new object, module.exports still points to the original. exports.c = 3 adds to the new object (not module.exports). require() returns module.exports → { a: 1 }.

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Q2: Singleton Behavior

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

// a.js
const counter = require('./counter');
counter.increment();
counter.increment();

// b.js
const counter = require('./counter');
console.log(counter.value()); // ?

Answer: 2

Why: Module is cached. a.js and b.js get the SAME object. Changes in a.js are visible in b.js.

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Q3: Circular Dependency

javascript// a.js
const b = require('./b');
module.exports = { name: 'A', b };

// b.js
const a = require('./a'); // what is 'a' here?
module.exports = { name: 'B', a };

// index.js
const a = require('./a');
console.log(a.name); // ?
console.log(a.b.name); // ?
console.log(a.b.a); // ?

Answer: 'A', 'B', {} (empty object — partial export of a.js when b.js loaded it)

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Q4: Dynamic require()

javascriptconst modules = ['fs', 'path', 'os'];
const loaded = modules.map(m => require(m));
console.log(loaded.map(m => typeof m)); // ?

Answer: ['object', 'object', 'object']

require() can be called dynamically with a variable. All three Node.js built-in modules are loaded and their types are 'object'.

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Q5: ESM Live Binding

javascript// counter.mjs
export let count = 0;
export const increment = () => { count++; };

// main.mjs
import { count, increment } from './counter.mjs';
console.log(count); // 0
increment();
increment();
console.log(count); // ?

Answer: 2 — ESM exports are live bindings. When increment() modifies count in counter.mjs, the imported count in main.mjs reflects that change.

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Q6: require() Caching

javascript// config.js
const config = { env: process.env.NODE_ENV || 'development' };
console.log('config loaded');
module.exports = config;

// app.js
const c1 = require('./config'); // ?
const c2 = require('./config'); // ?
console.log(c1 === c2); // ?

Answer: 'config loaded' prints ONCE. c1 === c2 → true. Module is cached after first require. Second require returns cached module.exports (same reference) without re-executing.

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Q7: What Does __dirname Give You?

javascript// /Users/alice/project/src/utils/helper.js
console.log(__dirname);
console.log(__filename);
console.log(process.cwd());

Answer:

• __dirname: /Users/alice/project/src/utils (directory of THIS file)
• __filename: /Users/alice/project/src/utils/helper.js (path of THIS file)
• process.cwd(): /Users/alice/project (where you ran node from — can differ!)

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Q8: Top-Level await in ESM

javascript// data.mjs
export const users = await fetch('https://api.example.com/users').then(r => r.json());
// This works in ESM! Top-level await is valid.

// main.mjs
import { users } from './data.mjs';
// main.mjs waits for data.mjs to fully load (including the await)

In CJS — must wrap in async function:

javascript// data.js
let users;
async function init() {
  users = await fetch('...').then(r => r.json());
}
await init(); // SyntaxError! Top-level await not allowed in CJS

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Q9: require() vs import() Performance

javascript// CJS require() — synchronous, blocks
const big = require('./large-module'); // blocks until loaded

// ESM dynamic import() — asynchronous, non-blocking
const big = await import('./large-module'); // non-blocking

For lazy loading or conditional imports, dynamic import() is better as it doesn't block.

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Q10: Module Resolution for node_modules

javascript// What's the lookup order for: require('express') ?
// (from /Users/alice/project/src/app.js)

// 1. /Users/alice/project/src/node_modules/express
// 2. /Users/alice/project/node_modules/express    ← usually here
// 3. /Users/alice/node_modules/express
// 4. /Users/node_modules/express
// 5. /node_modules/express
// 6. Error: Cannot find module 'express'