Frontend Interview Questions 2026: React, TypeScript & More

Frontend interviews in 2025 have evolved beyond "how does the event loop work." You'll face React architectural questions, TypeScript type system challenges, performance deep-dives, and often a system design component. This guide covers what's actually being asked and what a strong answer looks like. React Fundamentals Q: How does React's reconciliation algorithm work? What they're testing: Understanding of virtual DOM, fiber architecture, and why keys matter. Strong answer: React maintains a virtual DOM — a lightweight JavaScript representation of the actual DOM. When state or props change, React creates a new virtual DOM tree and diffs it against the previous one (reconciliation). It uses a heuristic O(n) algorithm that makes two assumptions: elements of different types produce different trees, and list items with stable keys can be identified across renders. The Fiber architecture (introduced in React 16) made reconciliation interruptible. Work is split into units of work (fibers),