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Browser & Web API Interview Questions

How browsers actually work: the render pipeline, storage options, service workers, Web Workers, CSP and security headers, Intersection Observer, Shadow DOM, and the platform APIs that come up in senior frontend interviews.

49 questions — free to read, with quizzes and AI-scored practice for members.

Browser & Web APIs49 questions

What are the key differences between localStorage and sessionStorage?junior What is HTTP and what are the most common HTTP methods?junior What is the difference between CSS `position: relative`, `absolute`, `fixed`, and `sticky`?junior What is the CSS box model and what are its four components?junior What is the DOM and how does JavaScript interact with it?junior How does CSS Flexbox work and what problems does it solve?junior Explain the Critical Rendering Path from HTML to pixel.mid How would you use the Intersection Observer API to implement lazy loading, infinite scroll, and impression tracking without scroll event listeners?mid Compare cookies, localStorage, and IndexedDB for client-side storage.mid How do you eliminate layout shift and invisible-text flash caused by web fonts, and what is the optimal loading strategy for a performance-critical site?mid How do you choose between cookies, localStorage, sessionStorage, and IndexedDB for different data persistence needs, and what are the security implications of each?mid Explain the differences between XSS and CSRF and how to prevent them.senior When should you choose WebSockets over polling or Server-Sent Events for real-time updates?senior How would you implement an Optimistic UI for a like button?senior How can you prevent Cross-Site Scripting (XSS) attacks in a web application?senior What is the difference between reflow and repaint in browser rendering?senior Why does a CORS error occur, and how can it be resolved?senior How would you design a client-side architecture for a chat application?senior How would you handle message ordering and delivery reliability in a chat application using WebSockets?senior How does React decide whether to update, mount, or unmount a component?senior How does event delegation improve performance in web applications?senior Explain the execution order of console logs in JavaScript with setTimeout and Promise.senior How does proper HTTP caching improve web performance?senior How does a Service Worker enable offline functionality in a PWA?senior How does the messaging app ensure message ordering and uniqueness?senior How would you implement a multi-layer cache strategy to efficiently fetch token prices for 1000 users accessing a widget simultaneously?senior How do Shadow DOM and Custom Elements enable widget isolation in a third-party SDK, and what are the limitations compared to iframes?senior Why is the first HTTPS request slower than subsequent ones?senior How does bidirectional virtualization improve performance in rendering large grids?senior How would you architect a news feed using modern web APIs?senior How does HTTP/3 (QUIC) improve upon traditional TCP for web communications?senior How would you design a URL shortening service to handle high read and write QPS efficiently?senior When does the cleanup function in useEffect run?senior How would you design a system to track transaction status across multiple blockchain networks?senior How can you ensure isolation of an embedded SDK widget from a host app's global styles and scripts?senior How would you design a system to offload CPU-intensive work from the main thread using Web Workers, and what are the serialization and communication trade-offs?senior How would you design a strategy to load, monitor, and sandbox third-party scripts (analytics, chat widgets, tag managers) without hurting Core Web Vitals?senior How do different client-side cache layers impact performance and data freshness?senior How does the stale-while-revalidate strategy improve user experience?senior How does virtualized grid rendering improve performance in a photo gallery with thousands of images?senior How does the event loop prioritize tasks in JavaScript?senior Explain how generational garbage collection works in JavaScript engines.senior Why is it beneficial to perform sorting and filtering on the server side for large datasets?senior How would you design and implement a strict Content Security Policy for a complex web application that uses third-party scripts and inline styles?senior How would you implement a feed using the provided API contract?senior How does a Service Worker manage network requests and caching in a web application?senior What are the pros and cons of using the 'Stale-While-Revalidate' cache strategy?senior What is CORS, why might it be blocked, and what is a preflight request?senior When should you use WebAssembly (WASM) and what is the integration pattern with React?senior

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