Full Stack with Node.js Rapid-Fire Questions &amp; Answers

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app.use mounts middleware for all HTTP methods and matches any path that starts with the given prefix, for example app.use('/api', router). app.get registers a handler only for GET requests on an exact route pattern. Use app.use for cross-cutting concerns like parsing and auth, and method-specific functions such as get, post, put and delete for endpoints.

What is a closure and how is it used in Node.js code?

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A closure is a function that remembers variables from the scope where it was created, even after that outer function has returned. In Node it is used for middleware factories like rateLimit({ max: 100 }), private counters, memoisation and module-level state. A risk is memory leaks when closures hold large objects longer than needed, for example inside long-lived event listeners.

What is the purpose of the util.promisify function?

Node.js Core and Event LoopMedium

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util.promisify converts a function that follows the Node callback convention, where the last argument is (err, result) => {}, into one that returns a promise. This lets you use async and await with older APIs. For example, const sleep = util.promisify(setTimeout) or promisify(fs.readFile). Many core modules now offer promise versions directly, such as fs/promises and dns/promises.

Explain the main HTTP methods and which are idempotent.

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GET reads a resource, POST creates or triggers an action, PUT replaces a resource fully, PATCH updates part of it and DELETE removes it. GET, PUT, DELETE, HEAD and OPTIONS are idempotent, meaning repeating the request has the same effect as doing it once. POST and usually PATCH are not, which is why payment APIs use idempotency keys for POST.

Explain semantic versioning and the meaning of ^ and ~ in package.json.

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Semantic versioning uses MAJOR.MINOR.PATCH: major for breaking changes, minor for new backward-compatible features, patch for bug fixes. A caret like ^4.18.2 allows any 4.x.x at or above 4.18.2. A tilde like ~4.18.2 allows only patch updates, 4.18.x. An exact version pins it. The lock file still decides what is actually installed.

What are the common strategies for API versioning?

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URL versioning like /api/v1/orders is the most common and easy to route and cache. Header versioning uses a custom header or Accept media type, keeping URLs clean but less visible. Query versioning like ?version=2 is simple but messy. Whatever you choose, version only for breaking changes, support old versions for a deprecation period and announce sunset dates.

What is package-lock.json and why should it be committed?

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package-lock.json records the exact resolved version and integrity hash of every installed package, including nested ones. Committing it makes installs reproducible across developer machines and CI, so a minor update in a transitive dependency cannot silently break production. In CI use npm ci, which installs strictly from the lock file and fails if it is out of sync with package.json.

Compare offset pagination and cursor pagination.

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Offset pagination uses page and limit, translated to LIMIT and OFFSET. It is simple and allows jumping to a page, but gets slow on large tables and can skip or repeat rows when data changes. Cursor pagination uses the last seen value, such as id or created_at, to fetch the next set with a WHERE clause. It is fast and stable, ideal for feeds and infinite scroll.

What is a compound index and why does field order matter?

Databases: MongoDB, SQL and RedisMedium

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A compound index covers multiple fields, such as { userId: 1, createdAt: -1 }. It supports queries that use a prefix of the fields, so it helps queries on userId alone or userId plus createdAt, but not createdAt alone. A good rule in MongoDB is ESR: put equality fields first, then sort fields, then range fields, to avoid in-memory sorts.

What happens if you forget to await a promise inside a try/catch?

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If you call an async function without await, the try/catch only covers the synchronous part, so a later rejection escapes it and becomes an unhandled rejection that can crash the process. The function also continues before the work finishes, causing race conditions like responding before data is saved. ESLint rules such as no-floating-promises in TypeScript help catch this.

What is a JWT and what are its three parts?

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A JSON Web Token is a compact, signed token used to carry claims between parties. It has three Base64URL parts separated by dots: header (algorithm and type), payload (claims like sub, role, exp) and signature. The signature proves the token was issued by the server and not modified. The payload is only encoded, not encrypted, so never put secrets in it.

What is the difference between process.nextTick and setImmediate?

Node.js Core and Event LoopMedium

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process.nextTick queues a callback to run immediately after the current operation, before the event loop continues to any phase, even before promise microtasks. setImmediate queues a callback for the check phase of the next loop iteration. Overusing nextTick recursively can starve I/O because the loop never moves on, so setImmediate is safer for deferring heavy work.

How do you design good REST endpoint URLs?

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Use plural nouns for resources and let methods express the action: GET /users, POST /users, GET /users/42, PATCH /users/42. Nest only for clear ownership, like GET /users/42/orders, and avoid deep nesting. Use lowercase kebab-case, query parameters for filtering, sorting and pagination, and version the API, for example /api/v1. Avoid verbs like /getUsers.

What is package.json and what is the difference between dependencies and devDependencies?

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package.json describes a project: name, version, scripts, entry point and dependencies. dependencies are packages needed at runtime, such as express or mongoose. devDependencies are only for development and build, such as jest, eslint or nodemon. Running npm ci --omit=dev in production installs only runtime packages, which keeps Docker images smaller and reduces attack surface.

What is a callback and what is callback hell?

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A callback is a function passed to another function to run when an asynchronous task finishes, following Node's error-first convention (err, data). Callback hell is deeply nested callbacks where each async step depends on the previous one, making code hard to read, handle errors in and maintain. Promises and async/await flatten the nesting and centralise error handling with catch or try/catch.

How do you scale a Node.js application horizontally?

Deployment, Scaling and PerformanceMedium

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Run multiple stateless instances behind a load balancer and add more as load grows, using autoscaling on CPU, memory or request rate. Keep state out of the process: sessions in Redis, files in object storage, jobs in a queue. Use Redis adapters for WebSocket broadcasting across instances, and make sure the database can handle the total connection count.

What is Redis and how is it used with Node.js backends?

Databases: MongoDB, SQL and RedisEasy

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Redis is an in-memory key-value store with data types like strings, hashes, lists, sets, sorted sets and streams. Node apps use it for caching database results, session storage, rate limiting counters, distributed locks, leaderboards, pub/sub between instances and job queues through BullMQ. Data lives in memory, so use TTLs and eviction policies and do not treat it as the only copy of important data.

What is the difference between authentication and authorisation?

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Authentication verifies who the user is, for example by checking a password, OTP or OAuth login. Authorisation decides what that verified user is allowed to do, such as whether they can delete an order or view admin reports. In an API, authentication failures return 401 and authorisation failures return 403. Both must be enforced on the server, never only in the frontend.

What is an index and how does it improve query performance?

Databases: MongoDB, SQL and RedisEasy

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An index is a data structure, usually a B-tree, that lets the database find rows without scanning the whole table or collection. Indexes speed up reads on filtered, sorted and joined fields but slow down writes and use storage. Create them based on real query patterns, check usage with EXPLAIN or explain('executionStats'), and remove unused ones.

What is the EventEmitter and how do you use it?

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EventEmitter from the events module implements the publish-subscribe pattern. Objects register listeners with on or once and trigger them with emit. Streams, HTTP servers and sockets all extend it. Always handle the 'error' event, because an emitted error without a listener crashes the process. Watch for memory leaks when listeners are added repeatedly; Node warns after 10 listeners by default.

What is the difference between REST and GraphQL?

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REST exposes multiple endpoints with fixed response shapes, uses HTTP caching easily and is simple to reason about. GraphQL exposes one endpoint where clients query exactly the fields they need, reducing over-fetching and multiple round trips, which helps mobile apps. GraphQL adds complexity: N+1 query risks needing DataLoader, harder caching, query cost limits and schema governance.

What is code coverage and is 100 percent coverage a good goal?

Testing and DebuggingEasy

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Code coverage measures how much code tests execute: lines, branches, functions and statements, reported by Jest with --coverage or c8. It shows untested areas but not whether assertions are meaningful. Chasing 100 percent leads to brittle tests of trivial code. A practical target is around 70 to 85 percent with strong coverage of business logic, error paths and security checks.

Why put Nginx in front of a Node.js application?

Deployment, Scaling and PerformanceEasy

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Nginx acts as a reverse proxy: it terminates TLS, serves static files efficiently, compresses responses, buffers slow clients, load balances across multiple Node instances and adds rate limiting and request size limits. This frees Node to focus on application logic. On cloud platforms a managed load balancer or ingress controller often plays the same role.

What is SQL injection and how do you prevent it in Node.js?

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SQL injection happens when user input is concatenated into a query, letting attackers change it, for example ' OR 1=1 --. Prevent it with parameterised queries, such as pool.query('SELECT * FROM users WHERE email = $1', [email]) in pg, or an ORM like Prisma or Sequelize that parameterises automatically. Also validate input and give the database user minimum privileges.

When would you choose NestJS or Fastify over Express?

Express.js FrameworkMedium

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Choose Fastify when raw performance and built-in schema validation matter; it is faster than Express thanks to optimised routing and serialisation. Choose NestJS for large teams that want structure: modules, dependency injection, decorators and first-class TypeScript, similar to Spring or Angular. Express still suits small to medium services and teams that prefer flexibility and a huge middleware ecosystem.

What is the Buffer class in Node.js?

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Buffer represents a fixed-length sequence of raw bytes stored outside the V8 heap. It is used for binary data such as files, images, TCP packets and crypto output. You create one with Buffer.from('text') or Buffer.alloc(size) and convert with toString('utf8' or 'base64'). Buffer.allocUnsafe is faster but may contain old memory, so fill it before use.

What is a REST API and what are its key constraints?

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REST is an architectural style where resources are identified by URLs and manipulated through standard HTTP methods. Key constraints: client-server separation, statelessness so each request carries all needed information, cacheable responses, uniform interface, layered system and optionally code on demand. In practice it means resource-based URLs, correct methods and status codes, and JSON representations.

What is Express.js and why is it widely used?

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Express is a minimal, unopinionated web framework for Node.js that adds routing, middleware and helpers for requests and responses on top of the http module. It is widely used because it is simple, flexible and has a large ecosystem of middleware for logging, auth, validation and security. Many teams also use it as the base for frameworks like NestJS.

What is the difference between res.send, res.json and res.end?

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res.send sends a response and sets Content-Type based on the argument: string as HTML, Buffer as binary, object as JSON. res.json always serialises to JSON and sets application/json, and respects app settings like json spaces. res.end ends the response with no extra processing, useful for empty responses such as 204. Call only one per request to avoid headers-already-sent errors.

Why is using async callbacks inside forEach a common bug?

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Array.forEach ignores the promise returned by an async callback, so it does not wait for any iteration and errors are not caught by the outer function. Code after the loop runs before the work is done. Use for...of with await for sequential processing, or map the items to promises and await Promise.all for parallel processing.

What is the difference between var, let and const?

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var is function-scoped, hoisted and initialised as undefined, and can be redeclared. let and const are block-scoped and are in a temporal dead zone until declared, so accessing them early throws an error. const prevents reassignment of the binding but not mutation of an object's contents. Modern Node code uses const by default and let when reassignment is needed.

How do you design a consistent error response format for an API?

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Use one structure everywhere, for example { "error": { "code": "VALIDATION_FAILED", "message": "Email is invalid", "details": [{ "field": "email" }], "requestId": "abc123" } }. Many teams follow RFC 9457 Problem Details with type, title, status and detail. Use machine-readable codes for clients, human messages for display, and never leak stack traces or SQL errors.

What are microtasks and macrotasks?

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Macrotasks are scheduled work like setTimeout, setInterval, setImmediate and I/O callbacks, handled phase by phase by the event loop. Microtasks are promise reactions and queueMicrotask callbacks; they run right after the current synchronous code finishes and before the next macrotask. In Node, process.nextTick callbacks run even before promise microtasks. A long microtask chain can delay timers and I/O.

Which HTTP status codes should a REST API commonly return?

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200 OK for successful reads and updates, 201 Created with a Location header for new resources, 204 No Content for deletes. 400 for invalid input, 401 when not authenticated, 403 when authenticated but not allowed, 404 not found, 409 conflict such as a duplicate email, 422 for validation errors in some APIs, 429 rate limited, 500 server error and 503 unavailable.

What are common ways to deploy a Node.js API in 2026?

Deployment, Scaling and PerformanceEasy

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Options include virtual machines like EC2 with PM2 and Nginx, containers on Kubernetes such as EKS, AKS or GKE, managed container services like AWS ECS Fargate, Cloud Run or Azure Container Apps, platforms like Render or Railway, and serverless functions on AWS Lambda. Choice depends on traffic patterns, team skills, cost and need for control.

How does HTTP caching with ETag and Cache-Control work for APIs?

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Cache-Control tells clients and CDNs how long a response is fresh, for example public, max-age=300, or private, no-store for user data. An ETag is a hash of the response; the client sends it back in If-None-Match, and if nothing changed the server returns 304 Not Modified with no body. Express generates weak ETags automatically for res.send responses.

What are streams in Node.js and what types exist?

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Streams process data in chunks instead of loading everything into memory. There are four types: Readable (fs.createReadStream, HTTP request), Writable (fs.createWriteStream, HTTP response), Duplex (TCP socket, both read and write) and Transform (zlib gzip, modifies data as it passes). Streams keep memory flat when serving large files, uploading media or processing big CSV logs.

How should passwords be stored in a database?

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Never store plain text or use fast hashes like MD5 or SHA-256 alone. Use a slow, salted password hashing algorithm such as bcrypt with a cost factor around 10 to 12, or Argon2id, which is the current recommendation. The library generates a unique salt per password. On login, compare using the library's compare function, which is timing-safe.

What is the difference between throwing an error and rejecting a promise inside an async function?

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Inside an async function they behave the same: throw new Error() causes the returned promise to reject with that error. Outside async functions, a throw inside a plain callback such as setTimeout is not caught by promise handlers and becomes an uncaught exception. Always reject with Error objects, not strings, so you keep a stack trace.

When should you embed documents versus reference them in MongoDB?

Databases: MongoDB, SQL and RedisMedium

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Embed when data is read together, owned by the parent and bounded in size, such as addresses inside a user or line items inside an order. Reference when the related data is large, grows without limit, is shared across many documents or changes independently, such as products referenced by orders or comments on a viral post. Remember the 16 MB document size limit.

How do you manage secrets such as database passwords and API keys in a Node.js app?

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Never commit secrets to Git. Load them from environment variables, using a .env file only for local development and keeping it in .gitignore. In production, use a secrets manager such as AWS Secrets Manager, Azure Key Vault, HashiCorp Vault or Kubernetes secrets with encryption. Rotate keys regularly, scan repos with tools like gitleaks and validate required variables at startup.

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

Testing and DebuggingEasy

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Unit tests check a single function or service in isolation with dependencies mocked; they are fast and numerous. Integration tests check several parts together, such as a route, service and a real test database. End-to-end tests run the full system like a user would, often through HTTP against a deployed environment. A healthy test pyramid has many unit tests and fewer E2E tests.

What is PM2 and when do you need it?

Deployment, Scaling and PerformanceEasy

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PM2 is a process manager for Node.js on virtual machines. It keeps apps running and restarts them on crashes, runs cluster mode across CPU cores, supports zero-downtime reloads, log management and startup scripts. In Docker or Kubernetes you usually do not need PM2, because the orchestrator restarts containers and you scale with replicas, so you run node directly.

What does helmet do in an Express application?

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helmet is middleware that sets secure HTTP response headers. It adds Content-Security-Policy, Strict-Transport-Security, X-Content-Type-Options nosniff, frame protection and removes X-Powered-By, reducing risks like clickjacking, MIME sniffing and some XSS attacks. It is one line, app.use(helmet()), but CSP usually needs tuning so your frontend scripts and assets still load.

Why is structured logging important and which libraries are used?

Testing and DebuggingEasy

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Structured logs are JSON lines with fields like level, timestamp, requestId, userId and message, so tools like ELK, Loki, Datadog or CloudWatch can search and aggregate them. pino is fast and popular, winston is flexible with many transports. Log at correct levels, include a correlation ID per request, and never log passwords, tokens, full card numbers or Aadhaar numbers.

How do you enable CORS in an Express app and why is it needed?

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Browsers block cross-origin requests unless the server sends CORS headers. Use the cors package: app.use(cors({ origin: ['https://app.example.com'], credentials: true })). Avoid origin '*' with credentials, and restrict methods and headers you allow. CORS is a browser protection, not a security layer for your API, so you still need authentication and authorisation.

Explain ACID properties of a database transaction.

Databases: MongoDB, SQL and RedisEasy

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Atomicity means all operations in a transaction succeed or none do. Consistency means the database moves from one valid state to another, respecting constraints. Isolation means concurrent transactions do not see each other's partial work, controlled by isolation levels. Durability means committed data survives crashes. Money transfers rely on ACID so a debit never happens without the matching credit.

How does the this keyword behave in arrow functions compared to regular functions?