Question 1

What are stream piping and the .pipe() method in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes very basic stream familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The pipe below was **actually run** on Node v24.19.0, delivering real data end to end, not described from documentation. ## 1. Why This Even Matters — A Story First A water pipe connects a tap directly to a basin — water flows from one to the

Question 2

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 3

What is the difference between readFile and createReadStream in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes very basic `fs` module familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The numbers below came from **actually reading a real 20MB file** both ways on Node v24.19.0, not a description of expected behavior. ## 1. Why This Even Matters — A Story First Moving a house's contents in one giant crate needs a crane

Question 4

What is the use of the 'os' module in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — no prior systems-programming background assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every value below was **read directly from this real machine**, not written as a generic placeholder example. ## 1. Why This Even Matters — A Story First A moving crew arriving at an unfamiliar building first checks a few basic fact

Question 5

How can you securely store and verify passwords in a Node.js application?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js security-focused phone screens — assumes very basic hashing familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The hash/verify pair below was **actually run** on Node v24.19.0, using only Node's built-in `crypto` module — no external package installed. ## 1. Why This Even Matters — A Story First A safe-deposit box's combination is ne

Question 6

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 7

What is the difference between unit, integration, and end-to-end tests for a Node.js API?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js testing-strategy interviews — assumes familiarity with the `node:test` runner and mocking questions' real proofs. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timing comparison below is **real, measured** output from actually running both a unit and an integration test with `node --test` — genuine milliseconds, not illustrative numbers.

Question 8

How would you implement caching in a Node.js application?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design interviews — assumes familiarity with the performance-techniques question's real cache-timing proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The TTL expiration below was **actually observed** — a real 150ms wait, a real check against the elapsed time, not an assumption that expiration "should" work. ## 1. Why This Even M

Question 9

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 10

How do you implement a custom Transform stream?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js streams interviews — assumes basic familiarity with `.pipe()` and readable/writable streams. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The chained transform output below was **actually run** — a genuine uppercase transformation and a genuine line count via a real `_flush()` call, not descriptions of documented behavior. ## 1. Why This E

NODE.JSDECODED.

Easy

What are stream piping and the .pipe() method in Node.js?

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

What is the difference between readFile and createReadStream in Node.js?

What is the use of the 'os' module in Node.js?

How can you securely store and verify passwords in a Node.js application?

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

What is the difference between unit, integration, and end-to-end tests for a Node.js API?

Medium

How would you implement caching in a Node.js application?

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

How do you implement a custom Transform stream?

What are Worker Threads in Node.js and when would you use them?

How do you manage sessions and authentication in a Node.js web application?

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

What is the 12-factor app methodology as applied to Node.js services?

Explain the differences between fork(), spawn(), and exec() in the child_process module.

Hard

Explain how memory management and Garbage Collection work in the V8 engine.

When would you use `child_process` module in Node.js?

What are memory leaks in Node.js and how do you detect them?

What is backpressure in Node.js streams and how do you handle it?

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

What are some common ways to improve performance in a Node.js application?

What are Single Executable Applications (SEA) in Node.js?

Explain the concept of 'Event Loop Pollution' and how to avoid it.