React Performance and Optimization Interview Questions
Performance and optimization come up in React interviews, especially for mid-level and senior roles.
React Performance and Optimization Interview Questions
Performance and optimization come up in React interviews, especially for mid-level and senior roles. Here are the common questions.
How do you optimize a React app?
Measure first with the Profiler, then stabilize references with useCallback and useMemo, wrap components with React.memo where needed, virtualize long lists, code split routes, and debounce expensive inputs.
What is the difference between useMemo and useCallback?
useMemo memoizes a value; useCallback memoizes a function reference. Use useMemo for expensive computations, useCallback for stable functions passed to memoized children.
What does React.memo do?
It wraps a component so it re-renders only when its props change by reference. Combine with useCallback and useMemo for stable prop references, so it does not re-render from new references on every parent render.
How do you optimize a long list in React?
Virtualize it with a library like react-window so only the visible items render. Also use stable keys, lazy-load images, and avoid inline function props that cause item re-renders.
What is the virtual DOM and how does it help?
The virtual DOM is React's in-memory description of the UI. React compares new and previous descriptions and updates the real DOM only where they differ, which makes updates efficient.
What is code splitting and why does it matter?
Breaking your bundle into chunks loaded on demand, usually via React.lazy and Suspense. It keeps the initial bundle small so users load faster, especially on slow networks.
The Takeaway
For React performance interviews, know how to optimize (measure, then memoize, virtualize, code split, debounce), useMemo vs useCallback, what React.memo does, list optimization, the virtual DOM, and code splitting. Start with measurement.
Measure first with the Profiler, then stabilize references with useCallback and useMemo, wrap components with React.memo where needed, virtualize long lists, code split routes with React.lazy, and debounce expensive inputs. Never optimize blindly.
useMemo memoizes a value; useCallback memoizes a function reference. Use useMemo for expensive computations, useCallback for stable functions passed to memoized children.
It wraps a component so it re-renders only when its props change by reference. Combine with useCallback and useMemo for stable prop references, so the memoized component does not re-render from new references on every parent render.
Virtualize it with a library like react-window so only the visible items render. Also use stable unique keys, lazy-load images, and avoid inline function props that cause item re-renders. Virtualization is the biggest win for large lists.
The virtual DOM is React's in-memory description of the UI. React compares new and previous descriptions and updates the real DOM only where they differ, a process called reconciliation. This makes updates efficient by minimizing DOM manipulation.
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