Question 1

What does the Node.js --watch flag do and how does it replace nodemon?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js developer-tooling interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The restart-with-a-new-PID below was **actually observed** from a real running `node --watch` process after a real file edit — not a description of documented behavior. ## 1. Why This Even Matters — A Story First Manually killing a terminal process and re-typing the

Question 2

What are the main security risks of using the 'eval()' function in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js security-focused phone screens — no prior security background assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every consequence below was **actually demonstrated by running real code**, not described as a hypothetical. ## 1. Why This Even Matters — A Story First Handing a stranger a blank check with your signature already on it is fun

Question 3

How do you run Node.js processes as a background service?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/DevOps phone screens — assumes very basic `child_process`/Unix process familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The detached-process survival below was **actually run** on this machine, with a real log file confirming the child kept working after its parent had already exited. ## 1. Why This Even Matters — A Story First A c

Question 4

What is the purpose of the util.promisify function in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes basic Promise and callback familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the success and rejection paths below were **actually executed** on Node v24.19.0, not assumed from the function's name. ## 1. Why This Even Matters — A Story First An old appliance with a two-prong plug still works perfectly —

Question 5

What is the role of the V8 JavaScript engine in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — no prior engine-internals knowledge assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The version number below was read directly from a running Node process, not quoted from memory. ## 1. Why This Even Matters — A Story First A car's engine and its dashboard, seatbelts, and cargo hold are built by different teams for dif

Question 6

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 7

How do you mock modules, timers, and network calls in Node.js tests?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js testing interviews — assumes familiarity with the `node:test` runner question's real pass/fail proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. All three mocking mechanisms below were **actually run** — a real 10-second timer genuinely fired in under 1ms, and a real method mock genuinely avoided a real network call, not descriptions of i

Question 8

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 9

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 10

What is Helmet and which HTTP security headers should a Node.js API set?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/Express security interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The header diff below is **real, measured** output from two actually-running Express servers — genuine response headers, not a description of what Helmet's documentation says it does. ## 1. Why This Even Matters — A Story First A house with no lock on the door, no pee

NODE.JSDECODED.

Easy

What does the Node.js --watch flag do and how does it replace nodemon?

What are the main security risks of using the 'eval()' function in Node.js?

How do you run Node.js processes as a background service?

What is the purpose of the util.promisify function in Node.js?

What is the role of the V8 JavaScript engine in Node.js?

Medium

How does the Node.js event loop work?

How do you mock modules, timers, and network calls in Node.js tests?

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

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

What is Helmet and which HTTP security headers should a Node.js API set?

What is the difference between "type": "module", .mjs, and .cjs?

How do you design liveness and readiness health-check endpoints?

How do you design REST API versioning in a Node.js service?

Hard

How do you handle circular dependencies in Node.js?

How does CommonJS `require()` an ES Module in modern Node.js?

What is diagnostics_channel and when would you use it?

How do you implement microservices communication in Node.js?

What is the 'vm' (Virtual Machine) module and when would you use it?

How do you secure your Node.js API endpoints?

How do you guarantee idempotency in a queue consumer?

What is prototype pollution and how do you prevent it in Node.js?