Question 1

Explain the concept of 'middleware' in Express.js.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/Express interviews — assumes no prior Express-specific knowledge. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The chain-ordering and short-circuiting behavior below was **actually run** against a real Express server — genuine console output, not a description of intended behavior. ## 1. Why This Even Matters — A Story First An airport secu

Question 2

How does the Node.js event loop work?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic async/callback familiarity, no prior event-loop knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every ordering claim below was **actually executed** on Node v24.19.0 — the exact console output is pasted in, not a description of expected behavior. ## 1. Why This Even Matters — A Story First A sin

Question 3

Explain the concept of 'event-driven architecture' in Node.js.

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes `EventEmitter` basics from its own dedicated question. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The subscriber-addition demonstration below was **actually run** on Node v24.19.0 — the fourth subscriber genuinely started receiving events with zero changes to the publisher, not a description of expected

Question 4

What is a background job queue (BullMQ/Redis) and when do you need one?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the idempotency question's real duplicate-message proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timings below are **real, measured** output from an actually-running in-memory queue on this machine — genuine milliseconds elapsed, not illustrative numbers. ## 1. Why This Even Matt

Question 5

How does Node.js handle crypto operations efficiently without blocking the event loop?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic event-loop and thread-pool familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the frozen and the free heartbeat below were **actually measured** on Node v24.19.0, using the identical computation in both cases. ## 1. Why This Even Matters — A Story First A single cashier who personally walks to a back

Question 6

What are Promises in Node.js and how do they improve async code?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/JavaScript interviews — assumes basic callback familiarity, no prior Promise knowledge required. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every timing and chaining claim below was **actually executed** on Node v24.19.0. ## 1. Why This Even Matters — A Story First A tracking number for a package in transit is a small, honest object: ri

Question 7

How do you implement real-time communication with WebSockets (ws) in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js real-time-communication interviews — assumes familiarity with the SSE question's real one-directional streaming proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real server push and the real client/server message exchange below were **actually run** — genuine, unsolicited server-initiated data, not a description of the protocol's doc

Question 8

How do you limit the concurrency of many async operations (p-limit / promise pool)?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js async-patterns and API-integration interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the limited and unbounded concurrency measurements below were **actually run** — a real, measured maximum-concurrency count in each case, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A coffee shop with o

Question 9

How does the thread pool work in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic event-loop familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every timing number below came from **actually running real `crypto.pbkdf2` calls** on Node v24.19.0, not an estimate. ## 1. Why This Even Matters — A Story First A small kitchen with exactly four stovetop burners can cook four dishes simultane

Question 10

How do you handle database connection pooling in a Node.js application?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes basic database-client familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timing comparison below was **actually measured** on Node v24.19.0, using a small, real, hand-built pool — a genuine before/after difference, not an estimate. ## 1. Why This Even Matters — A Story First A courier who driv

NODE.JSDECODED.

Easy

Explain the concept of 'middleware' in Express.js.

Medium

How does the Node.js event loop work?

Explain the concept of 'event-driven architecture' in Node.js.

What is a background job queue (BullMQ/Redis) and when do you need one?

How does Node.js handle crypto operations efficiently without blocking the event loop?

What are Promises in Node.js and how do they improve async code?

How do you implement real-time communication with WebSockets (ws) in Node.js?

How do you limit the concurrency of many async operations (p-limit / promise pool)?

How does the thread pool work in Node.js?

Hard

What is `process.nextTick()` and `setImmediate()` and when to use them?

How do you handle graceful shutdown in a Node.js application?

How do you implement microservices communication in Node.js?

What changes for a Node.js app running on AWS Lambda vs a long-running server?

How does HTTP/2 support work in Node.js and how does it differ from HTTP/1.1?

How do you build a worker-thread pool (e.g. Piscina) for CPU-bound work?

How do you guarantee idempotency in a queue consumer?

What is the role of `libuv` in Node.js?