Version 1.2.0

What's Changed

Added

• stale handler for match(): Both MatchHandlers<T> and SingleMatchHandlers<T> now accept an optional stale?: () => MaybePromise<MaybeCleanup> branch. It fires when all signals have a retained value but at least one Task signal is currently executing (isPending() === true). Routing precedence is nil > err > stale > ok; omitting stale falls back to ok, showing the retained value unchanged while the task re-fetches. Any cleanup returned by stale is registered on the owner and runs before the next handler dispatches — the right place to remove a refresh indicator or dim overlay. In React Query terms: nil maps to isLoading (no data yet); stale maps to isFetching with existing data.
• isSignalOfType<T>(value, type) utility: New exported function that replaces isObjectOfType for signal type guards. Checks value != null && value[Symbol.toStringTag] === type directly — zero string allocations, O(1). All eight internal is*() guards (isState, isMemo, isTask, isSensor, isSlot, isStore, isList, isCollection) now use it.
• DEEP_EQUALITY equality preset: New exported constant for deep structural comparison of plain objects and arrays. Uses Object.is as a fast path, then recursively compares array elements by index and own enumerable keys of plain-object records (Object.getPrototypeOf(v) === Object.prototype). Non-plain objects (class instances, Map, Set) are never structurally equal unless they are the same reference. Pass to the equals option to suppress propagation when a signal holding an object or array recomputes to a structurally identical value.
• DEFAULT_EQUALITY exported from index.ts: The ===-based equality preset was already used internally throughout the library but was not part of the public API. It is now exported, allowing callers to restore the default explicitly when composing or selectively overriding SignalOptions.

Changed

• isSignal uses a module-level Set with direct Symbol.toStringTag access: Previously allocated two strings per call via Object.prototype.toString.call(value).slice(8, -1) and scanned an inline array with Array.includes(). Now checks SIGNAL_TYPES.has(value[Symbol.toStringTag]) — one hash lookup, zero allocations, Set built once at module load.
• isRecord uses a prototype check instead of Object.prototype.toString: Previously Object.prototype.toString.call(value) === '[object Object]', which returns true for class instances without a custom Symbol.toStringTag. Now checks Object.getPrototypeOf(value) === Object.prototype, which excludes class instances. Affects createSignal and createMutableSignal: a class instance with no Symbol.toStringTag previously resolved to a Store; now it falls through to createState. Class instances are not plain records, so this is the correct behavior.
• isEqual / DEEP_EQUALITY cycle detection removed: Previously, the deep equality function in list.ts and store.ts allocated a WeakSet on every List.set() / Store.set() call, added both operands before recursing, and threw CircularDependencyError on a circular reference. The try/finally block cleaned up the WeakSet entries after each call. All of this is removed — the implementation is now plain recursion (deepEqual in graph.ts) with no allocations. Circular data causes a stack overflow rather than a thrown error. Signal values are expected to be plain JSON-like data; circular references are a programming error.
• Equality presets unified in graph.ts: DEFAULT_EQUALITY, SKIP_EQUALITY, and DEEP_EQUALITY are all defined in graph.ts alongside SignalOptions. Previously isEqual (the deep equality implementation) lived in list.ts as a private function and was imported by store.ts. Both files now import DEEP_EQUALITY from graph.ts; the CircularDependencyError import in list.ts is removed.

Deprecated

• isObjectOfType(value, type): Marked @deprecated. Allocates two strings per call (Object.prototype.toString.call() plus a template literal). Use isSignalOfType(value, type) for signal type guards instead. The function remains exported for backward compatibility and will be removed in a future release.
• isEqual: Deprecated alias for DEEP_EQUALITY. Previously the private deep equality implementation in list.ts, now re-exported from index.ts as a deprecated alias pointing to DEEP_EQUALITY in graph.ts. Replace all uses with DEEP_EQUALITY.

Fixed

• createScope effect leak on throw: Previously, if fn() threw after creating child effects, dispose was never created or registered with the parent owner — child effects leaked and continued running indefinitely. Now dispose is created before the try block and registered with prevOwner in the finally clause, so cleanup always executes regardless of whether fn() throws.
• list.replace() spurious dependency edge: Previously, calling replace() from inside an effect linked the item signal to the calling effect as a dependency (via the unguarded signal.get() equality check). The effect re-ran — and permanently acquired the dependency — after each replace() call. Now the check uses untrack(() => signal.get()), so no edge is created during the early-exit test.
• list.splice() signal corruption on same-key replace: Previously, splicing out an item and inserting a new item with the same content-based key left the key in keys but absent from signals — byKey() returned undefined silently. Now splice detects the key overlap and routes to change instead of an add+remove pair.
• match() err cleanup silently dropped on thrown errors: Previously, the catch branch called err([...]) without capturing the return value — cleanup functions or Promise<MaybeCleanup> returned by err were silently discarded (memory leak in the error path). Now out = err([...]) captures the return value for cleanup registration, matching the try-branch behavior.

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Full Changelog: v1.1.1...v1.2.0