Question 1

What is the difference between dependencies, devDependencies, and peerDependencies?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js phone screens — assumes basic npm familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The install behavior below came from **actually running** `npm install --save-dev` and a subsequent `npm install --omit=dev` on this machine, not a description of expected behavior. ## 1. Why This Even Matters — A Story First A moving company packs t

Question 2

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 3

What does NODE_ENV=production actually change and why does it matter?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js deployment and configuration interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the "Node core does nothing special" proof and the real Express behavior difference below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First A single word written on a sticky note ("PRODUCTION"

Question 4

What does the Node.js `--env-file` flag do, and does it replace dotenv?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js backend interviews — assumes basic `process.env` familiarity. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Everything below was **executed against Node v24.19.0** — the raw output is pasted in section 3. ## 1. Why This Even Matters — A Story First For years, every Node tutorial opened the same way: npm install dotenv, then

Question 5

How do you manage configuration in a Node.js application for different environments?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js interviews — assumes basic `process.env` familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The config-switching behavior below was **actually run** across three different `NODE_ENV` values on Node v24.19.0, including the failure case. ## 1. Why This Even Matters — A Story First A single stage play script works in two different the

Question 6

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js authentication and system-design interviews — assumes familiarity with the 12-factor app question's real stateless-processes proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the session revocation and the JWT tamper-detection below were **actually run** — a real 401 after a real logout, and a real signature-check rejection of a genu

Question 7

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

Accepted Answer

**Target Audience:** Engineers preparing for Node.js system-design and deployment-practices interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The config demo below was **actually run twice**, with and without environment variables set — real, different output, not a description of intended behavior. ## 1. Why This Even Matters — A Story First A recipe that says "add the amount of sal

Question 8

What does the events.once() helper do and how does it bridge EventEmitter and async/await?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js EventEmitter and async/await interoperability interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both the real event-timing match and the real error-rejection behavior below were **actually run** — genuine timestamps and a genuine caught rejection, not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First

Question 9

How do you monitor a Node.js application's memory usage in production?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js production-operations and system-design interviews — assumes familiarity with the memory-leaks question's real growth-pattern proof. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The memory numbers below are **real, captured output** from `node --expose-gc` on this machine — not illustrative placeholder numbers. ## 1. Why This Even Matters

Question 10

How do you containerize a Node.js app with a small, secure Docker image (multi-stage, distroless)?

Accepted Answer

**Target Audience:** Engineers preparing for Node.js deployment/DevOps-adjacent system-design interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The production-vs-full dependency size gap below was **actually measured** in this sandbox via real `npm install` runs. The full Docker image build itself could not be executed in this particular sandbox session (Docker Desktop's engine was not

NODE.JSDECODED.

Easy

What is the difference between dependencies, devDependencies, and peerDependencies?

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

What does NODE_ENV=production actually change and why does it matter?

What does the Node.js `--env-file` flag do, and does it replace dotenv?

Medium

How do you manage configuration in a Node.js application for different environments?

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

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

What does the events.once() helper do and how does it bridge EventEmitter and async/await?

How do you monitor a Node.js application's memory usage in production?

How do you containerize a Node.js app with a small, secure Docker image (multi-stage, distroless)?

What is the perf_hooks module and how do you measure performance?

How do you implement structured logging with pino or winston, and why avoid console.log in production?

Hard

How do you profile CPU usage with --prof, 0x, or clinic.js flame graphs?

How do you implement microservices communication in Node.js?

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

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

How do you guarantee idempotency in a queue consumer?

How do you add OpenTelemetry distributed tracing to a Node.js service?

How do you capture and analyze a heap snapshot to find a memory leak?