React useState Expert FAQs

Question 1

How do I update an object in state without mutating it?

Accepted Answer

Always create a new object using the spread operator, for example: setObj({ ...oldObj, property: newValue }). This ensures React detects the reference change and triggers a re-render.

Question 2

What is a lazy initializer in useState?

Accepted Answer

A lazy initializer is a function passed to useState that only runs during the initial mount. It is used to prevent expensive calculations from running unnecessarily on every subsequent render.

Question 3

Can I use useState inside a loop or conditional statement?

Accepted Answer

No, React Hooks must always be called at the top level of your component to ensure they are called in the exact same order every time the component renders.

Question 4

Why is my state not updating immediately after calling setState?

Accepted Answer

React state updates are snapshots; the new value is only available in the next render cycle. If you need the updated value immediately, calculate it as a variable or use the updater function pattern.

Question 5

When should I use useReducer instead of useState?

Accepted Answer

Use useReducer when state logic is complex, involves multiple sub-values, or when the next state depends on the previous state in intricate ways that benefit from centralized logic.

React useState Expert

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React useState Expert

主な機能

ユースケース