Notification semantics

Commits and delivery

Updates run through the current middleware chain. A value different by Object.is is committed immediately. The store captures that value and current subscriptions in a notification record, then drains records synchronously in FIFO order. No timers, microtasks, or batching are involved.

An unchanged reference, a same-value updater, or middleware that does not call next produces no notification. Middleware runs in outer-to-inner order; ordinary delivery remains inside next, before middleware code following it.

Reentrant writes

If a listener commits 2 while delivering 1, remaining selectors receive 1 before receiving 2. Selectors evaluate the captured state and receive next/previous selection arguments. getState() always reads the latest committed state, even while an older record is being delivered. Plain subscribe callbacks receive no arguments and are invalidation signals.

New subscriptions do not receive records captured before registration. Unsubscribe is immediate and idempotent: an inactive subscription is skipped even if present in a captured record. Registering the same callback twice creates independent subscriptions.

FIFO delivery avoids recursive stack growth; it does not prevent logically infinite update loops. Listener-driven updates still need a terminating condition.

Errors

A selector error during registration propagates immediately and registers nothing. Errors from a selector, equality function, or listener during delivery are collected. Other active subscriptions and queued records still run. After draining, the outermost notification call throws an AggregateError containing original thrown values in order. The queue is cleared and remains usable for the next update.

The state is already committed and is not rolled back. A failed selector/equality evaluation retains its previous selection baseline; a listener failure occurs after that baseline advances. Updater and middleware errors outside notification delivery propagate directly.

Explicit writes

set(value) treats functions as values; update(updater) treats its argument as computation. Both use the same middleware chain as setState(valueOrUpdater).