Why might reading state right after calling a setter show the old value, and how do functional setters help?

State updates, batching, and functional setters

Explains asynchronous-looking state updates, batching, functional setters, and avoiding stale values.

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React state setters schedule an update for a future render. The current render's state variable does not change immediately inside the same function. React may also batch multiple updates before rendering. When the next value depends on the previous value, I would use the functional setter form, such as setCount(previous => previous + 1), because it receives the latest queued state for that update.

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React state scheduling, batching, stale reads, functional updates, event handlers, and predictable state transitions.

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This is a practical bug source in interviews and real apps. Candidates often know how to call setState, but not how queued updates and stale values behave.
React state setters schedule an update for a future render. The current render's state variable does not change immediately inside the same function. React may also batch multiple updates before rendering. When the next value depends on the previous value, I would use the functional setter form, such as setCount(previous => previous + 1), because it receives the latest queued state for that update.
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I would be careful in event handlers, timers, promises, and subscriptions where closures can hold older state. If I call twice in the same handler, both calls may use the same captured . Functional setters compose correctly for incremental updates. For state objects, I would create a new object and merge intentionally rather than mutating the previous state or assuming React deeply merges objects.

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Expecting a state variable to change immediately after calling its setter.
Using captured state for repeated incremental updates.
Mutating state objects in place.
Adding effects only to patch unclear state transitions.

Why might reading state right after calling a setter show the old value, and how do functional setters help?

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