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Interviews / Frontend Engineering / Your UI freezes during large updates. How does React Fiber help?

Questions

A critical component is crashing in production. How would you debug it?
Q
This component makes duplicate API calls in development. How does Strict Mode help identify the bug?
Q C
This component is crashing the entire app. Implement an Error Boundary to gracefully handle the error.
Q C
This statically generated page is showing stale data. How do you fetch and display live data on the client?
Q C
This component is failing its test. Fix the component to make the test pass.
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This form doesn't reset correctly. Fix the bug and refactor to a more robust solution.
Q C
Your app's state management is a mess. How would Flux architecture help?
Q
This button should increment twice, but only increments once. How do you fix it?
Q C
This application suffers from 'prop drilling.' How do you fix it?
Q C
This component's UI is not updating. Why is direct state mutation problematic?
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This component's state logic is complex. Refactor it with `useReducer`.
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An expensive component is re-rendering unnecessarily. How would you fix this state management issue?
Q C
This list re-renders inefficiently. How does React's reconciliation algorithm explain why?
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This component re-renders unnecessarily due to context object reference changes. How do you fix it?
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An expensive component is re-rendering unnecessarily. How would you fix this state management issue?
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This optimized component re-renders unnecessarily. How do you fix it with `useCallback`?
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This large component is slowing down initial load. Implement code splitting.
Q C
This component performs expensive calculations unnecessarily. Optimize it with `useMemo`.
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This component re-renders unnecessarily. How do you prevent it?
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This data fetching component has a race condition. How do you fix it?
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This data fetching component is causing an infinite loop. How do you fix it?
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This component causes a hydration mismatch error. Why and how to fix it?
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This component has hardcoded text. Refactor it to support multiple languages.
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This component is not reusable. Refactor it using the composition pattern.
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This modal is clipped by its parent. Fix it using a React Portal.
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This component is breaking layout. Fix it using a React Fragment.
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This custom component is not forwarding its ref. How do you fix it?
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These components duplicate logic. Refactor them using a Higher-Order Component.
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These components duplicate logic. Refactor them using the Render Props pattern.
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This component manages loading state imperatively. Refactor it to use React Suspense.
Q C
This legacy class component mixes logic and UI. Refactor it to a modern functional component using Hooks.
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Why does this component log the old state value after calling a useState setter?
Q C
Clarify the core React concepts: Element, Component, and Node.
Q
Your team is evaluating React. How would you explain its core benefits?
Q
This component is crashing with a JSX syntax error. How do you fix it?
Q C
A child component is failing to update the parent's state. Why is this happening and how do you fix it?
Q C
This component is trying to modify a prop directly. How do you fix it?
Q C
Your UI freezes during large updates. How does React Fiber help?
Q
This component has accessibility ID issues. Fix it with `useId`.
Q C
This component flickers when resizing. Fix it using the correct effect hook.
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This component is crashing due to a 'Rules of Hooks' violation. How do you fix it?
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This `useEffect` is logging a stale value. How do you fix its dependency array?
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This component has issues managing a timer ID. Fix it with `useRef`.
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These components duplicate stateful logic. Refactor them using a custom Hook.
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This form uses an uncontrolled input. Refactor it to be a controlled component.
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Your UI freezes during large updates. How does React Fiber help?

conceptual React Internals Interactive Quiz
How Different Experience Levels Approach This
Junior Engineer
Surface Level

Fiber is some internal React thing that makes it faster.

Senior Engineer
Production Ready

Fiber is a complete rewrite of React's reconciliation algorithm that transforms rendering from a blocking, synchronous process into an interruptible, prioritized one:

The Problem Before Fiber

React’s old reconciliation was synchronous and uninterruptible. A 500ms update = 500ms UI freeze. No user input, no animations, nothing.

How Fiber Solves It

1. Interruptible Rendering Fiber breaks rendering into small units of work. It can pause, yield control to the browser (for user input, repaints), then resume. No more blocking the main thread.

2. Prioritization (Time Slicing) Not all updates are equal. Fiber assigns priorities:

  • User interactions (typing, clicking) → High priority
  • Background data fetches → Low priority

If a high-priority update arrives during low-priority work, React pauses the low-priority work and handles the urgent update first.

3. Concurrent Rendering Fiber enables working on multiple tasks simultaneously or interleaving them. This is the foundation for Suspense and Concurrent Mode.

Practical Benefits:

  • Smoother UX: Fewer freezes during heavy computations
  • Better interactions: Typing and clicking feel immediate
  • Better animations: Animation frames render on time
  • Foundation for features: Suspense, Concurrent Mode, Transitions
What Makes the Difference?
  • Context over facts: Explains when and why, not just what
  • Real examples: Provides specific use cases from production experience
  • Trade-offs: Acknowledges pros, cons, and decision factors

Practice Question

What is a key architectural improvement introduced by React Fiber?