Concurrent Store

What a store guarantees under concurrent React

You will learn

Where a blocking update lands while a Transition is still in flight
What a component mounting mid-Transition is allowed to display
When a promise in a store shows a fallback, and when it must not
What actually clears a rejected promise behind an error boundary

React’s documentation advises against suspending a render on a value read with useSyncExternalStore. A mutation to an external store cannot be marked as a Transition, so it triggers the nearest Suspense fallback and replaces content that is already on screen. It also notes that a store mutated during a Transition makes React redo that update as a blocking one.

Every panel below suspends on a store value on purpose. These are the six guarantees that makes possible.

Rebasing

A blocking update applies to what is on screen

Each letter is an action.

on screen∅store∅

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