Discuss 'useMemo' hook.
useMemo is a React Hook used to cache the result of a costly calculation between re-renders.
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Skipping expensive recalculations
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Skipping re-rendering of components
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Memoizing a dependency of another Hook
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Memoizing a function
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It should be pure, should take no arguments, and should return a value of any type.
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React will call this function during the initial render.
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React will return the same value on the next renders if the dependencies have not changed since the last render.
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Otherwise, it will call the calculatevalue function, return its result, and store it so it can be reused later.
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By default, React re-renders all of its children recursively when a component re-renders. useMemo can also help you optimise the performance of re-rendering child components.
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The list of all reactive values that are referenced inside the calculateValue code.
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Reactive values include props, state, and all the variables and functions declared directly inside your component body.
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Dependencies should also be memoised if they might be created on every render : const dependencyVariable={x:3, y:5}
Scenario Questions
0-2 years experience
- 1How would you use useMemo to avoid recalculating a filtered list each time the component re-renders?
- 2What happens if you omit the dependency array when calling useMemo?
- 3Can you write a short snippet where useMemo wraps a heavy computation and explain why it helps?
2-5 years experience
- 1We added a useMemo around a derived state, but the UI still feels sluggish. What could be causing that?
- 2Why might a useMemo that depends on an object prop not memoize as expected, and how would you fix it?
- 3When a component receives frequent prop updates, how do you decide which values belong in the useMemo dependency array?
5-8 years experience
- 1Design a reusable Table component that memoizes row rendering. What trade‑offs do you consider between useMemo and React.memo?
- 2During a code review you see a useMemo with a large dependency list causing unnecessary recomputations. How would you refactor it?
- 3At scale, how does excessive useMemo affect memory usage and what strategies would you use to monitor or limit its impact?
8+ years experience
- 1Our monorepo has many legacy components using useMemo inconsistently. How would you create a migration plan to standardize memoization practices across teams?
- 2When building a cross‑team shared data‑visualization library, how do you decide which calculations should be memoized with useMemo versus using a selector library like reselect?
- 3Discuss the impact of useMemo on server‑side rendering and hydration performance, and how you'd guide teams to handle it.
Follow-up Questions
- What are the risks of overusing useMemo?
- How does useMemo differ from useCallback in practice?
- Can you describe a scenario where removing useMemo actually improved performance?