Does React guarantee the order of state updates for both the same and different components?

React's State Update Order

React's state update process is designed to optimize performance by deferring updates and batching them together. However, the question arises whether React maintains the order of state updates for both the same component and different components.

Same Component

Yes, React guarantees that state updates for the same component will be applied in the same order they were called. This is maintained even when multiple calls to setState are made within a single event handler. React coalesces these updates into a single batch and applies them sequentially.

Different Components

React also maintains the order of state updates across different components. When a state update is triggered, it is added to a global queue. Updates are processed in the order they were added, ensuring that the state of dependent components is always consistent with the order of setState calls.

Example

In the given examples:

1. Same Component: Within the Container component, the state updates for a and b will be applied in the order they are called, ensuring that there is no intermediate state where a is false and b is true.
2. Different Components: In the SuperContainer and Container components, the state updates will also be applied in the order they are called. This means that there will be no intermediate state where a is false and b is true even though the updates are initiated in different components.

Batched Updates

Note that React introduces a concept called "batched updates." By default, state updates within event handlers are batched together to improve performance. This means that intermediate state changes may not be visible within the event handler itself. However, once the event handler completes, all the batched updates are applied at once, ensuring the correct order of state changes.

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