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Full Stack with Node.js Telephonic Interview Questions & Answers

30 questions for the telephonic interview stage — 18 easy, 10 medium, 2 hard.

About the telephonic interview round

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Full Stack with Node.js concepts clearly without a screen to point at.

Tips for the telephonic interview round

  • Keep your CV in front of you and be ready to explain every line on it.
  • Answer in short, clear sentences; there is no whiteboard, so structure matters.
  • Take the call somewhere quiet and confirm the time in advance.
  • Prepare one question to ask the interviewer at the end.
Solved — telephonic interviewStart practicing
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01

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

Asynchronous JavaScriptEasy
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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.
02

How does error-handling middleware work in Express?

Express.js FrameworkMedium
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Error-handling middleware has four parameters (err, req, res, next) and must be registered after all routes. When any middleware calls next(err) or throws in a synchronous handler, Express skips normal middleware and jumps to the error handler. In Express 4, async errors must be passed to next manually or with a wrapper; Express 5 forwards rejected promises automatically.
03

Explain debounce and throttle and where they matter on the backend.

Asynchronous JavaScriptMedium
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Debounce runs a function only after calls stop for a set time, merging bursts into one call. Throttle runs it at most once per interval. On the backend they help when batching cache invalidation, limiting webhook processing, writing metrics, or reacting to file watcher events. For API traffic you normally use a rate limiter instead, which is a server-side form of throttling.
04

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.
05

What are microtasks and macrotasks?

Asynchronous JavaScriptMedium
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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.
06

Explain the main HTTP methods and which are idempotent.

REST API DesignEasy
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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.
07

What is Mongoose and why use it with MongoDB?

Databases: MongoDB, SQL and RedisEasy
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Mongoose is an ODM for MongoDB in Node.js. It adds schemas with types, validation, defaults, middleware hooks such as pre save, virtuals, population of references and a convenient query API. This brings structure to schemaless MongoDB collections and reduces bugs. For very high-performance or simple cases, some teams use the native MongoDB driver directly to avoid the overhead.
08

What is a JWT and what are its three parts?

Authentication and SecurityEasy
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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.
09

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.
10

How would you design authentication for a system of ten microservices?

Authentication and SecurityHard
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Centralise login in an identity service or provider such as Keycloak, Auth0 or Cognito that issues short-lived JWTs signed with RS256. The API gateway validates tokens, and each service verifies the signature using the public keys from a JWKS endpoint, so no shared secret is needed. Use service-to-service tokens or mTLS for internal calls and keep authorisation checks inside each service.
11

What is the difference between var, let and const?

Asynchronous JavaScriptEasy
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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.
12

How do you manage configuration for dev, staging and production environments?

Deployment, Scaling and PerformanceEasy
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Follow the twelve-factor approach: keep config in environment variables, not code. Load and validate them at startup with a schema, for example zod or envalid, and fail fast if any are missing. Use .env files only locally, and store production values in the platform's secret store or config maps. Keep the same Docker image across environments and change only configuration.
13

What is the built-in node:test runner and when would you use it over Jest?

Testing and DebuggingMedium
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Node.js ships a stable test runner in the node:test module with describe, it, mocks, coverage and watch mode, run with node --test. It needs no extra dependency and works natively with ES modules, which Jest still handles with extra configuration. Jest or Vitest offer richer snapshot testing, mocking and ecosystem. For small libraries or ESM-first services, node:test is a lightweight choice.
14

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.
15

What is soft delete and what are its trade-offs?

Databases: MongoDB, SQL and RedisMedium
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Soft delete marks a record as deleted with a flag or deletedAt timestamp instead of removing it, so data can be restored and audited. Trade-offs: every query must filter out deleted rows, unique constraints need care, tables grow over time and privacy laws such as India's DPDP Act may require real deletion of personal data on request. Use partial indexes and periodic purging.
16

What is the difference between REST and GraphQL?

REST API DesignMedium
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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.
17

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.
18

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.
19

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

Asynchronous JavaScriptMedium
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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.
20

Why is using async callbacks inside forEach a common bug?

Asynchronous JavaScriptEasy
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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.
21

What is Node.js and why is it used for backend development?

Node.js Core and Event LoopEasy
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Node.js is a JavaScript runtime built on the V8 engine that runs JavaScript outside the browser. It uses a single-threaded event loop with non-blocking I/O, so one process can handle thousands of concurrent connections cheaply. It is popular for APIs, real-time apps and microservices because teams can use one language across frontend and backend and reuse a huge npm ecosystem.
22

What is the EventEmitter and how do you use it?

Node.js Core and Event LoopEasy
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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.
23

Why do you want to work as a Node.js backend developer?

Deployment, Scaling and PerformanceEasy
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I enjoy building the systems behind products: APIs, data models and integrations. Node.js lets me use JavaScript across the stack and ship features quickly, and I like solving problems around performance and reliability. In my projects I built a REST API with authentication, MongoDB and Docker deployment, and I want to grow by working on real traffic and learning system design from experienced engineers.
24

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

Express.js FrameworkEasy
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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.
25

Explain OAuth 2.0 and the Authorization Code flow with PKCE.

Authentication and SecurityHard
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OAuth 2.0 lets an app access resources on a user's behalf without seeing their password, for example social login with an existing account. In Authorization Code with PKCE, the app creates a code verifier and sends its hash to the authorisation server. The user logs in and consents, the app receives a code and exchanges it plus the verifier for tokens. PKCE stops intercepted codes being reused.
26

Tell me about a time a database change caused an issue and what you learned.

Databases: MongoDB, SQL and RedisMedium
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Situation: I added a NOT NULL column with a default to a large MySQL table during business hours. Task: the migration locked the table and checkout slowed down. Action: I rolled back, rewrote it as add nullable column, backfill in batches, then add the constraint, and ran it at low traffic after testing on a production-size copy. Result: no downtime, and we added a migration review checklist.
27

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.
28

What is the difference between req.params, req.query and req.body?

Express.js FrameworkEasy
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req.params holds route parameters from the path, such as id in /users/:id. req.query holds query string values like ?page=2&sort=name, used for filtering and pagination. req.body holds the parsed request payload of POST, PUT or PATCH and needs a parser such as express.json(). All are user input and must be validated before use.
29

What is a callback and what is callback hell?

Asynchronous JavaScriptEasy
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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.
30

What is the difference between app.use and app.get?

Express.js FrameworkEasy
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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.

Frequently asked questions

What happens in the Full Stack with Node.js telephonic interview round?

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Full Stack with Node.js concepts clearly without a screen to point at.

How do I clear the telephonic interview round?

Keep your CV in front of you and be ready to explain every line on it. Answer in short, clear sentences; there is no whiteboard, so structure matters. Take the call somewhere quiet and confirm the time in advance.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

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