Senior Backend Developer (Node.js) Interview Questions: Complete Guide

Introduction

This comprehensive guide contains 30 essential interview questions covering advanced Node.js backend development. These questions are designed to help senior backend developers prepare for interviews by covering key concepts in the Event Loop, Streams, System Design, and Performance Optimization. Each question includes a detailed answer, rarity assessment, and difficulty rating.

As a senior developer, you are expected to deeply understand the single-threaded nature of Node.js and how to build scalable, high-performance systems around it.

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Advanced Node.js Concepts (10 Questions)

1. Explain the Node.js Event Loop in detail. What are the different phases?

Answer: The Event Loop is what allows Node.js to perform non-blocking I/O operations despite being single-threaded. It offloads operations to the system kernel whenever possible.

• Phases:
  1. Timers: Executes callbacks scheduled by setTimeout() and setInterval().
  2. Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration.
  3. Idle, Prepare: Internal use only.
  4. Poll: Retrieves new I/O events; executes I/O related callbacks. This is where the loop blocks if there are no other tasks.
  5. Check: Executes callbacks scheduled by setImmediate().
  6. Close Callbacks: Executes close callbacks (e.g., socket.on('close', ...)).

Rarity: Very Common Difficulty: Hard

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2. What is the difference between process.nextTick() and setImmediate()?

Answer:

• process.nextTick(): It is not part of the Event Loop. It fires immediately after the current operation completes, but before the Event Loop continues. It has higher priority than setImmediate(). Excessive use can block the Event Loop (starvation).
• setImmediate(): It is part of the Check phase of the Event Loop. It runs after the Poll phase.

Rarity: Common Difficulty: Medium

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3. How does Node.js handle concurrency if it is single-threaded?

Answer: Node.js uses an event-driven, non-blocking I/O model.

• Main Thread: Executes JavaScript code (V8 engine).
• Libuv: A C library that provides the Event Loop and a thread pool (default 4 threads).
• Mechanism: When an asynchronous operation (like file I/O or network request) is initiated, Node.js offloads it to Libuv. Libuv uses its thread pool (for file I/O, DNS) or system kernel async mechanisms (for network). When the operation completes, the callback is pushed to the Event Loop queue to be executed by the main thread.

Rarity: Common Difficulty: Medium

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4. Explain Streams in Node.js and their types.

Answer: Streams are objects that let you read data from a source or write data to a destination in continuous chunks. They are memory efficient because you don't need to load the entire data into memory.

• Types:
  1. Readable: For reading data (e.g., fs.createReadStream).
  2. Writable: For writing data (e.g., fs.createWriteStream).
  3. Duplex: Both readable and writable (e.g., TCP sockets).
  4. Transform: Duplex streams where the output is computed based on input (e.g., zlib.createGzip).

Rarity: Common Difficulty: Medium

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5. What is Backpressure in Streams and how do you handle it?

Answer: Backpressure occurs when a Readable stream pushes data faster than the Writable stream can process it. If not handled, memory usage will spike until the process crashes.

• Handling:
  • .pipe(): The easiest way. It automatically manages backpressure by pausing the Readable stream when the Writable stream's buffer is full and resuming it when it drains.
  • Manual: Listen to the drain event on the Writable stream and pause/resume the Readable stream manually.

Rarity: Medium Difficulty: Hard

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6. How does the cluster module work?

Answer: Since Node.js is single-threaded, it runs on a single CPU core. The cluster module allows you to create child processes (workers) that share the same server port.

• Master Process: Manages the workers.
• Worker Processes: Each runs an instance of your application.
• Benefit: Allows you to utilize all available CPU cores, increasing throughput.

Rarity: Common Difficulty: Medium

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7. Worker Threads vs Cluster module: When to use which?

Answer:

• Cluster: Creates separate processes. Each has its own memory space and V8 instance. Best for scaling HTTP servers (I/O bound).
• Worker Threads: Creates threads within a single process. They share memory (via SharedArrayBuffer). Best for CPU-intensive tasks (e.g., image processing, cryptography) to avoid blocking the main Event Loop.

Rarity: Medium Difficulty: Hard

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8. How do you handle uncaught exceptions and unhandled promise rejections?

Answer:

• Uncaught Exception: Listen to process.on('uncaughtException', cb). It's usually best to log the error and restart the process (using a process manager like PM2) because the application state might be corrupted.
• Unhandled Rejection: Listen to process.on('unhandledRejection', cb).
• Best Practice: Always use try/catch blocks and .catch() on promises.

Rarity: Common Difficulty: Easy

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9. What is the role of package-lock.json?

Answer: It describes the exact tree that was generated, such that subsequent installs are able to generate identical trees, regardless of intermediate dependency updates. It ensures that your project works exactly the same way on every machine (CI/CD, other developers).

Rarity: Common Difficulty: Easy

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10. Explain the concept of Middleware in Express.js.

Answer: Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle (next).

• Tasks: Execute code, modify req/res objects, end the request-response cycle, call the next middleware.
• Order: They execute sequentially in the order they are defined.

Rarity: Common Difficulty: Easy

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System Design & Architecture (10 Questions)

11. How would you design a real-time chat application?

Answer:

• Protocol: WebSockets (using socket.io or ws) for full-duplex communication.
• Backend: Node.js is ideal due to its event-driven nature handling many concurrent connections.
• Scaling:
  • Redis Pub/Sub: If you have multiple server instances, a user connected to Server A needs to send a message to a user on Server B. Redis Pub/Sub acts as a message broker to broadcast messages across servers.
• Database:
  • Messages: NoSQL (MongoDB/Cassandra) for high write throughput.
  • Users: Relational (PostgreSQL) or NoSQL.

Rarity: Very Common Difficulty: Hard

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12. Microservices in Node.js: Communication patterns.

Answer:

• Synchronous: HTTP/REST or gRPC. Good for simple request/response.
• Asynchronous: Message Queues (RabbitMQ, Kafka, SQS). Good for decoupling services and handling load spikes.
• Event-Driven: Services emit events, others listen.

Rarity: Common Difficulty: Medium

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13. How do you handle Distributed Transactions (Saga Pattern)?

Answer: In microservices, a transaction might span multiple services. ACID is hard to guarantee.

• Saga Pattern: A sequence of local transactions. Each local transaction updates the database and publishes an event or message to trigger the next local transaction in the saga.
• Compensation: If a local transaction fails, the saga executes a series of compensating transactions that undo the changes made by the preceding local transactions.

Rarity: Medium Difficulty: Hard

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14. Explain the Circuit Breaker pattern.

Answer: It prevents an application from repeatedly trying to execute an operation that's likely to fail (e.g., calling a down microservice).

• States:
  • Closed: Requests pass through.
  • Open: Requests fail immediately (fast fail) without calling the service.
  • Half-Open: Allows a limited number of requests to check if the service has recovered.

Rarity: Medium Difficulty: Medium

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15. How do you secure a Node.js API?

Answer:

• Helmet: Sets various HTTP headers to secure the app.
• Rate Limiting: Use express-rate-limit to prevent DDoS/Brute force.
• Input Validation: Use libraries like Joi or Zod.
• Authentication: JWT or OAuth2.
• CORS: Configure properly to allow only trusted domains.
• NoSQL Injection: Sanitize inputs against MongoDB injection.

Rarity: Common Difficulty: Medium

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16. What is Serverless and how does it fit with Node.js?

Answer: Serverless (e.g., AWS Lambda) allows you to run code without provisioning or managing servers.

• Node.js fit: Node.js is excellent for serverless due to its fast startup time (cold start) and lightweight nature.
• Use Cases: API endpoints, event processing (S3 uploads), scheduled tasks.

Rarity: Medium Difficulty: Medium

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17. Explain GraphQL vs REST. When to use GraphQL?

Answer:

• REST: Multiple endpoints, over-fetching or under-fetching of data.
• GraphQL: Single endpoint, client asks for exactly what it needs.
• Use GraphQL: When you have complex data requirements, multiple clients (web, mobile) needing different data shapes, or to reduce network round trips.

Rarity: Common Difficulty: Medium

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18. How do you implement Caching in Node.js?

Answer:

• In-Memory: node-cache (good for single instance, but data is lost on restart and not shared).
• Distributed: Redis (industry standard).
• Strategies: Cache-Aside, Write-Through.
• HTTP Caching: Use ETag, Cache-Control headers.

Rarity: Common Difficulty: Medium

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19. Database Connection Pooling.

Answer: Opening a new database connection for every request is expensive.

• Pooling: Maintains a cache of database connections that can be reused.
• Node.js: Libraries like pg (PostgreSQL) or mongoose handle pooling automatically. You need to configure pool size based on your workload and DB limits.

Rarity: Medium Difficulty: Medium

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20. How do you handle file uploads in Node.js?

Answer:

• Multipart/form-data: The standard encoding for file uploads.
• Libraries: multer (middleware for Express), formidable, busboy.
• Storage: Don't store files in the server filesystem (statelessness). Upload to cloud storage like AWS S3. Stream the file directly to S3 to avoid loading it into memory.

Rarity: Common Difficulty: Medium

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Performance & Testing (10 Questions)

21. How do you debug a memory leak in Node.js?

Answer:

• Symptoms: Increasing memory usage over time (RSS), eventual crash.
• Tools:
  • Node.js Inspector: --inspect flag, connect with Chrome DevTools.
  • Heap Snapshots: Take snapshots and compare them to find objects that are not being garbage collected.
  • process.memoryUsage(): Monitor programmatically.

Rarity: Common Difficulty: Hard

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22. Profiling Node.js Applications.

Answer: Profiling helps identify CPU bottlenecks.

• Built-in Profiler: node --prof app.js. Generates a log file. Process it with node --prof-process isolate-0x...log.
• Clinic.js: A suite of tools (clinic doctor, clinic flame, clinic bubbleprof) to diagnose performance issues.

Rarity: Medium Difficulty: Hard

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23. Explain the "Don't Block the Event Loop" rule.

Answer: Since there is only one thread, if you execute a long-running synchronous operation (e.g., heavy calculation, synchronous file read, complex regex), the Event Loop stops. No other requests can be processed.

• Solution: Partition the calculation (setImmediate), use Worker Threads, or offload to a microservice.

Rarity: Very Common Difficulty: Easy

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24. Unit Testing vs Integration Testing in Node.js.

Answer:

• Unit Testing: Testing individual functions/modules in isolation. Mock dependencies. (Tools: Jest, Mocha, Chai).
• Integration Testing: Testing how modules work together (e.g., API endpoint + Database). (Tools: Supertest).

Rarity: Common Difficulty: Easy

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25. What is TDD (Test Driven Development)?

Answer: A development process where you write the test before the code.

1. Write a failing test (Red).
2. Write the minimum code to pass the test (Green).
3. Refactor the code (Refactor).

Rarity: Medium Difficulty: Medium

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26. How do you handle logging in a production Node.js app?

Answer:

• Don't use console.log: It's synchronous (blocking) when writing to a terminal/file in some cases and lacks structure.
• Use a Logger: Winston, Bunyan, or Pino.
• Structure: JSON format for easy parsing by log management tools (ELK Stack, Datadog).
• Levels: Error, Warn, Info, Debug.

Rarity: Common Difficulty: Easy

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27. Explain Semantic Versioning (SemVer).

Answer: Format: MAJOR.MINOR.PATCH (e.g., 1.2.3).

• MAJOR: Incompatible API changes.
• MINOR: Backwards-compatible functionality.
• PATCH: Backwards-compatible bug fixes.
• ^ vs ~: ^1.2.3 updates to <2.0.0. ~1.2.3 updates to <1.3.0.

Rarity: Common Difficulty: Easy

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28. What are Environment Variables and how do you manage them?

Answer:

• Purpose: Configuration that varies between environments (Dev, Staging, Prod) like DB URLs, API keys.
• Usage: process.env.VARIABLE_NAME.
• Management: .env files (using dotenv package) for local dev. In production, set them in the OS or container/platform settings.

Rarity: Common Difficulty: Easy

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29. How do you deploy a Node.js application?

Answer:

• Process Manager: PM2 (keeps app alive, handles restarts, logs).
• Reverse Proxy: Nginx (handles SSL, static files, load balancing) -> Node.js App.
• Containerization: Docker (standard).
• Orchestration: Kubernetes.
• CI/CD: GitHub Actions, Jenkins.

Rarity: Common Difficulty: Medium

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30. What is Event Emitter?

Answer: The events module is the core of Node.js event-driven architecture.

• Usage:

  const EventEmitter = require('events');
  const myEmitter = new EventEmitter();
  myEmitter.on('event', () => console.log('an event occurred!'));
  myEmitter.emit('event');

• Many core modules extend it: fs.ReadStream, net.Server, http.Server.

Rarity: Common Difficulty: Easy