easyrequire and module.exports
What does `module.exports` vs `exports` mean when you write only `exports = { ready: true }` in a module?
// flags.js
exports = { ready: true };
// app.js
console.log(require('./flags'));NODEJS · PRACTICE
NodeJS interview questions.
Node puts JavaScript on the server with a single-threaded event loop and a large streaming and process API surface.
519questions
103easy
216medium
200hard
Event-loop phases, the difference between setImmediate and process.nextTick, backpressure on streams, cluster and worker threads, and how to avoid blocking the loop.
Where it shows up: API services, BFF layers, build tooling, and serverless functions across most JavaScript-first companies.
Sample NodeJS questions
Twenty real NodeJS questions from the library, code snippet included. Nothing here is paraphrased for search engines. It is the same text a signed-in user sees.
easyCommonJS vs ES modules
What happens when this CommonJS file runs in Node?
// index.js (no "type": "module")
import os from 'os';
console.log(42);mediumprocess.nextTick vs setImmediate
Inside a Promise .then() callback, you schedule both a process.nextTick and a queueMicrotask, then resolve another promise. What is the output order?
Promise.resolve().then(() => {
console.log('then');
process.nextTick(() => console.log('nextTick'));
queueMicrotask(() => console.log('microtask'));
Promise.resolve().then(() => console.log('inner then'));
});easycallbacks error-first
Why does this code crash the process instead of logging the error?
const fs = require('fs');
try {
fs.readFile('nope.txt', (err, data) => {
if (err) throw err;
console.log(data);
});
} catch (e) {
console.log('caught:', e.message);
}mediumreadable vs writable streams
What does cork() followed by uncork() on a Writable stream achieve when the stream implements both _write and _writev?
const w = new Writable({
write(c, e, cb) { cb(); },
writev(chunks, cb) { console.log('writev', chunks.length); cb(); }
});
w.cork();
w.write('a'); w.write('b'); w.write('c');
process.nextTick(() => w.uncork());easypromises and async await
What does this print?
async function run() {
const r = await [Promise.resolve(1), Promise.resolve(2)][0];
console.log(r);
}
run();easyprocess.env variables
What does this snippet print when the environment variable `DEBUG` was never set?
console.log(process.env.DEBUG);easycallbacks error-first
What happens when you run this error-first callback code?
const fs = require('fs');
fs.readFile(__filename, 'utf8', (err) => {
if (err) throw err;
console.log('done');
console.log(data);
});mediumWorker threads basics
What does this worker-related snippet log, and why?
const { isMainThread } = require('worker_threads');
console.log(isMainThread);
// run as: node app.jseasycallbacks error-first
What is the problem with this error-first callback usage?
const fs = require('fs');
fs.readFile('config.json', 'utf8', (err, data) => {
console.log(data.length);
if (err) console.log(err);
});mediumreadable vs writable streams
A Transform stream is created with new Transform({ readableHighWaterMark: 5, writableHighWaterMark: 10, ... }). What is true about its two highWaterMark values?
const { Transform } = require('stream');
const t = new Transform({
readableHighWaterMark: 5,
writableHighWaterMark: 10,
transform(c, e, cb) { cb(); }
});
console.log(t.readableHighWaterMark, t.writableHighWaterMark);mediumchild_process spawning
Using child_process.spawn('echo', ['$HOME', '&&', 'whoami']) with the default options, what does the child receive as arguments?
const cp = require('child_process');
const r = cp.spawnSync('echo', ['$HOME', '&&', 'whoami']);
console.log(JSON.stringify(r.stdout.toString()));easyprocess.env variables
A teammate logs process.env.PORT and is surprised it prints 3000 (a number in their .env) as the string "3000". Why, and what is the safe fix to use it as a number?
// .env contains: PORT=3000
const port = process.env.PORT;
console.log(typeof port); // ?
const server = app.listen(port);mediumBuffer for binary data
What does Buffer.concat([a, b], totalLength) do when totalLength is smaller than the combined length of the buffers?
const x = Buffer.from([1, 2, 3]);
const y = Buffer.from([4, 5, 6]);
const z = Buffer.concat([x, y], 4);
console.log(z.length, [...z]);easypath and __dirname
Given a Node script run as `node /app/src/server.js`, what does this log?
// file: /app/src/server.js
const path = require('path');
console.log(path.join(__dirname, '..', 'config.json'));easyrequire and module.exports
What does this CommonJS module export?
exports = { a: 1 };
module.exports.b = 2;mediumprocess.nextTick vs setImmediate
Why does the relative order of setTimeout(fn, 0) and setImmediate(fn) inside the main module become deterministic when both are placed inside an I/O callback?
const fs = require('fs');
fs.readFile(__filename, () => {
setTimeout(() => console.log('timeout'), 0);
setImmediate(() => console.log('immediate'));
});mediumgraceful shutdown handling
What is the danger in this SIGTERM handler when a deployment sends repeated signals or shutdown stalls?
let shuttingDown = false;
process.on('SIGTERM', async () => {
await drainQueue();
await server.close();
process.exit(0);
});mediummemory leak debugging
Which of these is a genuine, common cause of a slow memory leak in long-running Node services?
const emitter = new EventEmitter();
function handle(req, res) {
emitter.on('tick', () => res.end('ok')); // per request
}easypromises and async await
Inside the loop, what does `await` do to the timing of this code?
for (const url of urls) {
const res = await fetch(url);
console.log(res.status);
}Sign in to answer
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NodeJS topics covered
48 distinct NodeJS topics are tagged across the library. These are the real topic labels stored on the questions, not a hand-written list.
- Worker threads basics (24)
- V8 garbage collection (23)
- child_process spawning (22)
- streams backpressure internals (21)
- event loop phases (19)
- memory leak debugging (18)
- async_hooks execution context (18)
- graceful shutdown handling (17)
- readable vs writable streams (16)
- process.nextTick vs setImmediate (15)
- promises and async await (15)
- CommonJS vs ES modules (14)
- crypto hashing signing (14)
- piping streams (12)
- Buffer for binary data (12)
- async_hooks for context (12)
- native addons N-API (12)
- custom EventEmitter classes (12)
- npm and package.json (11)
- libuv thread pool (11)
- SharedArrayBuffer and Atomics (11)
- V8 hidden classes (11)
- callbacks error-first (10)
- cluster for multi-core (10)
- Buffer pool and allocation (10)
- C++ bindings and the bridge (10)
- --max-old-space-size tuning (10)
- AbortController cancellation (10)
- microtasks vs timer phase (10)
- worker_threads memory sharing (9)
- fs file reading writing (9)
- require and module.exports (9)
- DNS and IO blocking the loop (9)
- http server basics (8)
- process.nextTick queue order (8)
- cluster and socket sharing (8)
- perf_hooks timing (8)
- path and __dirname (7)
- process.env variables (7)
- HTTP keep-alive agents (7)
- backpressure handling basics (7)
- JSON parse stringify (3)
- npm scripts (3)
- dotenv config (3)
- setTimeout setInterval (1)
- console and debugging (1)
- EventEmitter basics (1)
- handling uncaught errors (1)
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