Full Stack Developer Interview Questions & Answers
Frequently asked full stack developer interview questions across JavaScript, React, Node, APIs and databases — with concise answers. Practise them in a free AI mock interview.
Practise these in a free AI mock interview1. What is the difference between state and props in React?
Props are read-only data passed from a parent to a child. State is data a component owns and can change over time. Updating state re-renders the component; props change only when the parent passes new values.
2. Explain the JavaScript event loop.
JavaScript is single-threaded; the event loop lets it handle async work. Synchronous code runs on the call stack; promise callbacks wait in the microtask queue and timer callbacks in the macrotask queue, running when the stack is empty. Microtasks run before the next macrotask.
3. What happens when you type a URL and press Enter?
DNS resolves the domain to an IP, a TCP (and TLS) connection is made, the browser sends an HTTP request, the server responds with HTML, then the browser parses HTML/CSS/JS, builds the DOM and renders — fetching linked assets along the way.
4. How do you prevent SQL injection?
Never concatenate user input into queries. Use parameterised queries / prepared statements (or an ORM), validate and sanitise input, and give the database account least privilege.
5. What makes an API RESTful?
REST uses HTTP verbs (GET/POST/PUT/DELETE) on resources identified by URLs, returning representations (usually JSON). A RESTful API is stateless, resource-oriented, and uses correct status codes.
6. SQL vs NoSQL — when do you choose each?
SQL databases are relational with fixed schemas and strong consistency — great for structured, related data and transactions. NoSQL offers flexible schemas and horizontal scale — great for large, evolving or unstructured data. Choose by data shape and scale needs.
7. What are React hooks? Give examples.
Hooks let function components use state and lifecycle features: useState for local state, useEffect for side effects, useContext for shared data, useMemo/useCallback for performance. They must be called at the top level, not conditionally.
8. How would you debug and fix a slow web page?
Measure first (Lighthouse/DevTools). Then compress/lazy-load assets, code-split JS, cache (CDN + HTTP caching), reduce re-renders, add DB indexes for slow queries, and avoid blocking the main thread.
9. What is CORS?
Cross-Origin Resource Sharing — a browser rule that blocks a page from calling a different origin unless that server returns the right Access-Control-Allow-Origin headers. You fix it on the server, not the browser.
10. Authentication vs authorization?
Authentication verifies who you are (login). Authorization decides what you are allowed to do (roles/permissions). You authenticate first, then authorize each action.
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