Question 1

How do you enable gzip/Brotli compression for HTTP responses?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/Express performance interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real byte counts below are **actual, measured** raw wire bytes, captured via a client that bypasses automatic decompression specifically to measure what's genuinely sent — not illustrative numbers. ## 1. Why This Even Matters — A Story First Shipping a box stuf

Question 2

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 3

How do you handle CORS (Cross-Origin Resource Sharing) in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js/Express and web-security interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The header behavior below was **actually observed** from a real running Express server with the real `cors` middleware — genuine response headers, not a description of intended behavior. ## 1. Why This Even Matters — A Story First A nightclub bouncer checks

Question 4

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 5

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 6

How do you replicate __dirname in ES Modules using import.meta.url?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js ESM-migration interviews — assumes basic familiarity with CommonJS's `__dirname`. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the crash and the fix below were **actually run** in a real ESM file — a genuine `ReferenceError` with its exact message, and a genuinely correct reconstructed path, not illustrative output. ## 1. Why This Even

Question 7

What is the difference between blocking and non-blocking I/O in Node.js?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic `fs` module and event-loop familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The tick counts and timings below came from **actually running both versions** on Node v24.19.0 against a real 50MB file, not a description of expected behavior. ## 1. Why This Even Matters — A Story First A single toll-booth work

Question 8

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 9

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 10

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.

NODE.JSDECODED.

Easy

How do you enable gzip/Brotli compression for HTTP responses?

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

How do you handle CORS (Cross-Origin Resource Sharing) in Node.js?

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

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

How do you replicate __dirname in ES Modules using import.meta.url?

What is the difference between blocking and non-blocking I/O in Node.js?

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

Medium

What is AbortSignal.timeout and how do you cancel async work cleanly?

What are some best practices for structuring a Node.js project?

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

Explain how the EventEmitter class works under the hood.

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

What is the purpose of the AbortController in modern Node.js?

What are `EventEmitter`s in Node.js?

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

Hard

What is diagnostics_channel and when would you use it?

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

How do you implement microservices communication in Node.js?

What is the purpose of the HTTP Agent in Node.js?

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

How do you achieve concurrency in Node.js since it's single-threaded?

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

What are the Web Streams API (ReadableStream/WritableStream) in Node.js and how do they differ from node:stream?