Question 1

What is useState and when do you use it?

Accepted Answer

Hook that adds local reactive state to a function component. Returns [value, setState]. Updates queue a re-render. Prefer multiple state variables when unrelated.

Question 2

Why can setState appear async / batched?

Accepted Answer

React batches updates for performance; reading state right after setState shows the old value. Use functional updates setCount(c => c + 1) when next value depends on previous. In React 18+, more updates batch automatically.

Question 3

When do you lift state up?

Accepted Answer

When multiple siblings need the same data or a parent must orchestrate them. Single source of truth prevents desync. Don't lift everything to the app root prematurely.

Question 4

useState vs useReducer?

Accepted Answer

useState for simple values; useReducer for complex transitions or related multi-field updates. Reducers make event→state transitions explicit and testable. Prefer when next state logic is intricate.

Question 5

Storing derived data in state — why is it a smell?

Accepted Answer

Duplicated state drifts out of sync; derive during render or with useMemo instead. Keep minimal state; compute filtered lists from source + filter query.

Question 6

How do you update objects/arrays in state immutably?

Accepted Answer

Create new references via spread/map/filter; never mutate nested fields in place. Immutability enables correct change detection and time-travel debugging. Libraries like Immer help.

Question 7

Design state for a multi-step wizard form.

Accepted Answer

Hold step index + form draft in parent or reducer; persist optionally; validate per step. Avoid losing draft on step change. Consider URL step sync for deep links.

State React Interview Questions

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