Interview questions and answers for experienced candidates

Interview questions for people with work experience: career moves, leadership, tough situations and role-specific topics, with model answers.

Fresher questions · Experienced questions · Salaries · Company reviews

Angular HTTP, RxJS and state

• What are common RxJS operators you use, and what do they do?
• Explain the difference between switchMap, mergeMap, concatMap and exhaustMap.
• What are HTTP interceptors, and what are they used for?
• How do you handle errors from HTTP calls?
• How do you prevent memory leaks from subscriptions?
• How would you manage state in a large Angular application?
• What is the difference between hot and cold Observables, and what does shareReplay do?
• What is the resource API (and httpResource) in Angular, and how does it compare with using HttpClient and RxJS?
• What are Subject, BehaviorSubject and ReplaySubject, and when would you use each?

Angular components and templates

• What is the component lifecycle? Describe the main hooks.
• What is the difference between the constructor and ngOnInit?
• What is ViewEncapsulation?
• What are template reference variables?
• How does content projection (ng-content) work?
• What do @if, @for and @switch do, and why was this syntax added?
• What is @defer, and when would you use it?

Angular directives, pipes and DI

• How do you create a custom pipe? What is the difference between pure and impure pipes?
• What does the async pipe do, and why is it recommended?
• How do you build a custom attribute directive?
• What is the difference between providedIn root and providing a service in a component?
• What is the inject() function, and how does it compare with constructor injection?
• What are injection tokens, and when would you use InjectionToken?
• Explain the hierarchical injector. How does Angular decide which instance to give?
• What are providers like useClass, useValue, useFactory and useExisting?
• What are host directives and when are they useful?

Angular forms

• How do you write a custom validator?
• What are async validators and when do you need one?
• What are FormArray and dynamic forms?
• What is the difference between valueChanges and statusChanges, and how do you use them safely?
• What do touched, dirty and pristine mean?
• How would you build a reusable custom form control?
• What are typed forms, and why do they matter?

Angular performance and SSR

• What are the main ways to improve Angular app performance?
• Why is calling a method in a template expression a performance problem?
• What is NgOptimizedImage and what does it give you?
• What is server-side rendering (SSR), and why would you use it?
• What is hydration in Angular?
• What is the difference between SSR, SSG (prerendering) and CSR, and how do you choose?
• How do you analyse and reduce the bundle size?
• How do you find and fix a slow Angular screen?
• What is virtual scrolling and when should you use it?

Angular practical coding

• Coding exercise: implement sorting and filtering in Angular.
• Coding exercise: create a shopping cart in Angular.
• Coding exercise: implement JWT authentication in an Angular app.
• Coding exercise: build a reusable modal component in Angular.
• Coding exercise: implement lazy loading in Angular.
• Coding exercise: create a file upload component in Angular.
• Coding exercise: build an autocomplete dropdown in Angular.
• Coding exercise: implement infinite scrolling in Angular.
• Coding exercise: debug unnecessary component rendering in Angular.

Angular routing

• What is lazy loading, and how do you do it with routes?
• What are route guards? Give examples.
• What is a resolver, and when would you use one?
• How do child routes and nested router outlets work?
• How do you handle 404 pages and redirects?
• What are the preloading strategies for lazy routes?
• How would you implement role-based access in an Angular app?

Angular signals and change detection

• How do signal inputs, outputs and model() work?
• What is change detection in Angular?
• What is the OnPush change detection strategy?
• What is Zone.js and what does it do?
• What is zoneless change detection, and what must you change in your code to use it?
• What is linkedSignal, and when would you use it instead of computed?
• How do you avoid common problems with effects?
• How do toSignal and toObservable help mix signals and RxJS?
• What does ExpressionChangedAfterItHasBeenCheckedError mean and how do you fix it?
• What is the difference between markForCheck and detectChanges?

Angular testing, tooling and architecture

• How do you test a component that calls an API?
• How do you test route guards, pipes and directives?
• What does ng lint and a formatter give you, and what other tooling do Angular teams use?
• How do you handle environment-specific settings in Angular?
• How would you structure a large Angular application?
• How do you handle security in an Angular app (XSS, tokens, CSRF)?
• How do you upgrade Angular to a new major version safely?
• What is the difference between a micro frontend approach and a monolithic Angular app, and when would you use micro frontends?

Authentication and security

• How do you write authentication middleware in Express?
• Where should you store a JWT in the browser?
• What is an ownership check and why is it important?
• What do helmet and rate limiting protect against?
• What is CSRF and when do you need protection?
• What is XSS and how do you prevent it in a MERN app?
• How do access tokens and refresh tokens work together?
• How would you implement role-based access control (RBAC)?
• What would you do about leaked or compromised secrets?
• How do you keep dependencies secure?

Closing the interview

• Do you have any questions for us?

Customer service

• How would you handle an angry customer?

Driving and delivery

• What do you do before starting a delivery route?

Express basics and middleware

• What is the purpose of express.Router?
• What is the difference between app.use and app.get?
• How does error-handling middleware work in Express?
• What is the recommended project structure for an Express API?
• How do you serve static files with Express?
• What changed in Express 5 compared with Express 4?
• How do you structure middleware order in a production Express app?
• What does trust proxy do and why does it matter behind nginx?

HR and behavioural

• Tell me about yourself.
• What are your strengths and weaknesses?
• Why do you want to work for our company?
• Where do you see yourself in five years?
• Why should we hire you?
• Tell me about a time you faced a difficult situation at work or study.
• What are your salary expectations?
• Why did you leave your last job (or why is there a gap in your resume)?
• How do you handle a disagreement with your manager or a team member?
• Why are you looking to change jobs now?
• Describe a time you led a team or took responsibility beyond your role.

MERN architecture and scaling

• What is the MERN stack and what does each letter do?
• What are the pros and cons of the MERN stack?
• When would you choose SQL over MongoDB for a project?
• How would you structure a MERN app that has to grow?
• What is the difference between a monolith and microservices, and what suits a small MERN team?

MERN: connecting React and the API

• How do you set up a Vite proxy for development?
• How do you serve a React build from Express?
• How do you handle authentication state in a MERN frontend?
• How do you handle loading, error and empty states for API data in React?
• How do you build a dashboard with filters and pagination backed by the API?
• How do you share validation and types between frontend and backend?
• How do you handle file uploads from React to Express?

MongoDB and Mongoose

• What is the difference between embedding and referencing documents?
• What does populate do in Mongoose?
• What are indexes and why do they matter?
• How does validation work in Mongoose, and is it enough?
• What is the aggregation pipeline?
• What is the difference between find().lean() and a normal Mongoose document?
• How do you prevent NoSQL injection?
• What are MongoDB transactions and when do you need them?
• How would you model and query a large collection for good performance?

Node.js basics

• What is the difference between CommonJS and ES modules in Node?
• What is the event loop?
• What is the difference between process.nextTick, setImmediate and setTimeout?
• How would you use more than one CPU core in Node?

Node.js modules, files and async

• What is the difference between Promise.all, allSettled, race and any?
• Why is it a problem to await inside a loop, and what is the alternative?
• What are streams in Node and when do you use them?
• What is an EventEmitter?
• How do you handle unhandled promise rejections?
• What is backpressure in streams?
• How do worker threads differ from child processes?

Node.js testing, performance and deployment

• What is a reverse proxy and why put nginx in front of Node?
• How do you deploy a MERN app to a VPS?
• How do you do graceful shutdown in Node?
• How do you log properly in a production Node app?
• How do you improve the performance of a Node API?
• What is a health check endpoint?
• How do you find a memory leak in a Node app?
• How would you set up CI/CD for a Node app?
• What is the difference between horizontal and vertical scaling for a Node API?

Office and accounts

• How do you prioritise when you have many tasks and short deadlines?
• How do you make sure your work is accurate?

Python advanced features

• What is a generator and what does yield do?
• What is a decorator?
• What is the difference between an iterator and an iterable?
• What are threads, processes and asyncio, and when do you use each?
• How do async and await work in Python?
• What is functools.lru_cache?
• What are the walrus operator and match statement?
• What is a metaclass?
• What are __slots__ and when are they useful?
• How would you write a decorator that takes arguments?
• How do you profile and speed up slow Python code?
• What is the difference between __getattr__ and __getattribute__?

Python basics

• What is the difference between Python 2 and Python 3?
• What is the Python GIL?

Python classes and OOP

• How does Python do encapsulation, since it has no private keyword?
• What is the @property decorator?
• What are dunder (magic) methods?
• What is polymorphism and duck typing?
• What is the difference between composition and inheritance?
• What is the MRO and how does super() work with multiple inheritance?
• What are abstract base classes and protocols?

Python control flow and functions

• What are *args and **kwargs?
• Why is a mutable default argument dangerous?
• What is a lambda function?
• Explain scope and the LEGB rule.
• What are list comprehensions?
• What is recursion, and what are its limits in Python?
• What are first-class functions and closures?
• What are keyword-only and positional-only parameters?

Python data types and strings

• What is the difference between shallow copy and deep copy?
• What is the difference between mutable and immutable types, and why does it matter?
• How does Python manage memory?
• What is type hinting and does Python enforce it?
• How does string interning and integer caching affect is comparisons?
• What does hashable mean?

Python data, testing and performance

• What is NumPy and why is it faster than lists?
• What is mocking, and how do you do it in Python tests?
• What are pytest fixtures?
• How do you handle missing data in pandas?
• How do you build a small web API in Python?
• How do you automate files and folders with Python?
• How do you handle large files that do not fit in memory?
• How do you make a Python project production ready?
• How do you decide between Python and another language for a service?
• What is the difference between multiprocessing and concurrent.futures?

Python errors and files

• What is a context manager?
• What does "EAFP" mean?
• How do you log properly in Python?
• What is exception chaining?
• How do you handle several exceptions at once, and what are exception groups?

Python lists, dictionaries and sets

• What is the time complexity of common list, dict and set operations?
• How do you merge two dictionaries?
• What is defaultdict and Counter?
• What is the difference between a list and a deque?
• What is unpacking and how can you use it?
• How do you iterate with index and with two lists together?
• What is a generator expression and how does it differ from a list comprehension?
• How would you find the top N items in a large collection efficiently?
• What are dataclasses and when do you use them?

Python modules, packages and environments

• What does if __name__ == "__main__" do?
• What is the difference between import styles and circular imports?
• How do you structure a medium-sized Python project?
• How do you manage and pin dependencies for reproducible builds?
• How does Python find modules when you import?

REST API design and errors

• What makes a request idempotent, and which methods are?
• How do you validate input in an Express API?
• How do you implement pagination in an API?
• How should an API return errors?
• How do you version an API?
• How do you design an API that is safe to retry?
• What is rate limiting and how do you apply it?
• How do you handle file uploads in an Express API?

React 19 and architecture

• What are React Server Components, in simple terms?
• What is the difference between client-side rendering, server-side rendering and static generation?
• What are error boundaries and how do you use them?
• What are portals and when do you use them?
• What is new in React 19 that a developer should know?
• How would you structure folders and code in a large React project?
• How do you decide between building a single-page app and using a framework like Next.js?
• What security issues should a React developer think about?
• How do you deploy a React app to production?

React JSX, components and props

• What is a fragment and why would you use it?
• What are default props and how do you set default values today?
• What is prop drilling, and how can you avoid it?
• What is the difference between controlled and uncontrolled components?
• What is component composition, and why does React prefer it to inheritance?
• What is a higher-order component, and have hooks replaced it?

React basics

• What are the main benefits and drawbacks of React?
• What is one-way data flow in React?
• What happens when you call setState (or a state setter) in React?
• Explain what reconciliation is and how keys help.

React data fetching and state libraries

• What does TanStack Query (React Query) give you over useEffect and fetch?
• What is the difference between server state and client state?
• When do you need Redux, Zustand or another state library?
• What is Redux Toolkit and why is it recommended over plain Redux?
• What is optimistic UI and how can you build it?
• How do you handle authentication tokens in a React app securely?
• How would you structure API calls in a large React app?

React hooks and effects

• How do you fetch data in a React component with useEffect?
• What are race conditions in effects and how do you prevent them?
• When should you NOT use useEffect?
• What is the difference between useEffect and useLayoutEffect?
• How does useReducer work?
• What is a stale closure in React, and how do you fix it?
• What is useEffectEvent and what problem does it solve?
• How do you subscribe to an external store safely?
• What is the difference between useTransition and useDeferredValue?

React performance

• What is the difference between useMemo and useCallback?
• What is the React Compiler, and does it replace useMemo and useCallback?
• How do you lazy-load components in React?
• How do you render a very long list efficiently?
• Why can inline objects and functions as props hurt memoisation?
• How would you find and fix a slow React page?
• What is Suspense, and how does it relate to concurrent rendering?
• How do keys affect performance and correctness beyond lists?

React practical coding

• Coding exercise: implement sorting and filtering in React.
• Coding exercise: create a shopping cart in React.
• Coding exercise: implement JWT authentication in a React app.
• Coding exercise: implement lazy loading in React.
• Coding exercise: create a file upload component in React.
• Coding exercise: build an autocomplete dropdown in React.
• Coding exercise: implement infinite scrolling in React.
• Coding exercise: debug unnecessary component rendering in React.

React refs, context and custom hooks

• What is forwardRef and how does it change in React 19?
• What is the difference between useRef and useState?
• How can Context cause performance problems, and how do you reduce them?
• How do you write a useDebounce hook?
• How do you pass data between sibling components?
• What is useImperativeHandle?
• How would you design a reusable custom hook with a good API?
• What patterns exist for sharing logic between components, and how do they compare?

React routing and forms

• How do protected routes work?
• What are nested routes and layout routes?
• Why keep filters and pagination in the URL?
• When would you use React Hook Form or a similar library?
• How do you validate a form properly?
• What do React 19 form Actions change?

React state and events

• What is "lifting state up"?
• Why can a state update seem to use an old value, and how do you fix it?
• What is state batching in React?
• When should you use useReducer instead of useState?
• How do you decide where state should live?
• What does it mean to derive state, and why is it better than syncing it with an effect?

React testing, accessibility and TypeScript

• Why does Testing Library prefer getByRole over test ids?
• How do you mock API calls in component tests?
• What is accessibility (a11y) in React and what are the basics?
• How do you use TypeScript with React components?
• What makes a good test strategy for a React app?

SQL SELECT, filtering and sorting

• What is the difference between WHERE and HAVING?
• How do you paginate results in SQL?
• How do you find the second highest salary?
• What is the difference between CHAR and VARCHAR?
• How does CASE work?
• Why is SELECT * discouraged in production code?
• What is SQL injection and how do you prevent it?
• What is the difference between UNION and UNION ALL?

SQL aggregates and GROUP BY

• How do you count rows per category including categories with zero rows?
• How do you calculate a running total or percentage of total?
• How do you get the top N rows per group?
• What happens with NULL in GROUP BY and in aggregates?
• How do you pivot rows into columns in SQL?
• What are GROUPING SETS, ROLLUP and CUBE?
• How would you compute month-over-month growth?

SQL and database basics

• What is the order in which SQL clauses are executed?
• What is normalisation and why is it done?
• What is the difference between OLTP and OLAP?

SQL changing data safely

• What is an upsert?
• How do you add a column or change a table?
• What are default values and auto-increment/identity columns?
• How do you delete millions of rows safely?
• What is the difference between a transaction rollback and a backup restore?

SQL indexes, transactions and performance

• What is the difference between a clustered and a non-clustered index?
• What is a composite index and why does column order matter?
• How do you check how a query runs?
• Why might an index not be used?
• What are transaction isolation levels?
• What is a deadlock and how do you avoid it?
• What is the N+1 query problem?
• What is a view and when would you use one?
• What are stored procedures, functions and triggers?
• How would you optimise a slow query?
• What is the difference between optimistic and pessimistic locking?
• What are replication, sharding and partitioning?
• How do you choose between SQL and NoSQL for a new project?
• What is connection pooling and why does it matter?

SQL joins

• Why does a missing join condition create duplicate or huge results?
• What is the difference between putting a condition in ON and in WHERE for a LEFT JOIN?
• How do you join more than two tables?
• What is the difference between EXISTS and IN, and JOIN?
• What join algorithms do databases use?
• How would you find the latest record per group, such as each customer latest order?

SQL keys and table design

• What are constraints in SQL?
• Natural key or surrogate key: which is better?
• How do you model a many-to-many relationship?
• What does ON DELETE CASCADE do and when is it risky?
• What is a soft delete and what are its trade-offs?
• When might you denormalise?
• How do you design a schema for a feature like orders and products?
• How do you change a schema safely on a live system?

SQL subqueries, CTEs and set operations

• What is a correlated subquery?
• What is a CTE and why use one?
• What is a recursive CTE and when is it used?
• What is the difference between a CTE, a view and a temporary table?
• What do INTERSECT and EXCEPT do?
• Why is NOT IN dangerous with NULLs?
• What is a materialised view?

SQL window functions

• What is a window function and how is it different from GROUP BY?
• What is the difference between ROW_NUMBER, RANK and DENSE_RANK?
• What do PARTITION BY and ORDER BY do inside OVER?
• What do LAG and LEAD do?
• How do you calculate a running total?
• How do you find consecutive days (gaps and islands)?
• What does NTILE do and where is it used?
• Can you filter on a window function result directly in WHERE?

Sales

• How do you handle rejection or a customer saying no?

Skilled trades

• Tell me about a job you completed that you are proud of.

Technology

• What is the difference between frontend and backend development?
• How do you approach debugging a problem you have never seen before?
• Tell me about a mistake you made at work and what you learned.

Warehouse and logistics

• What safety precautions do you follow in a warehouse?
• How do you keep stock records accurate?