Questions
5 of 10
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useState
useReducer
useMemo
useContext
useCallback
useEffect
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Why are they called reducer functions?
Difficulty: 5/10
useReducer, reducer pattern, state management
- 1
They’re called “reducers” because the idea comes directly from the concept of a reduce operation—like the one in JavaScript’s Array.prototype.reduce.
- 2
A Redux reducer function is the same idea as this 'reduce callback' function! It takes a 'previous result' (the state), and the 'current item' (the action object), decides a new state value based on those arguments, and returns that new state.
Scenario Questions
0-2 years experience
- 1How would you use a reducer function with the useReducer hook to toggle a boolean flag in a component?
- 2What happens if you return a new state object that doesn't include all previous properties inside a reducer?
2-5 years experience
- 1We have a form with multiple fields and you implemented useReducer. If the form stops updating after adding a new field, how would you debug the reducer?
- 2When deciding between useState and useReducer for managing a list of items, what trade‑offs would you consider?
5-8 years experience
- 1In a large dashboard you’re using multiple nested reducers. How would you structure them to avoid unnecessary re‑renders and keep the state predictable?
- 2If a reducer starts causing performance bottlenecks because it runs on every keystroke, what strategies could you apply to mitigate the impact?
8+ years experience
- 1Your team is migrating a legacy codebase that uses many setState calls to a centralized reducer pattern. What architectural considerations and migration steps would you propose?
- 2When multiple teams share a common reducer library, how do you handle versioning and breaking changes without disrupting production?
Follow-up Questions
- Can you walk me through a simple reducer implementation for toggling a boolean?
- Why is immutability important in a reducer function?
- What would you do if a reducer threw an error during dispatch?