chore(signals): add default undefined type (#578)

* add default undefined type

* add changeset

.changeset/new-emus-check.md

@@ -0,0 +1,7 @@
+---
+"@preact/signals": minor
+"@preact/signals-react": minor
+"@preact/signals-core": minor
+---
+
+Allow for passing no argument to the signal and the type to be automatically inferred as `T | undefined`

packages/core/src/index.ts

@@ -392,7 +392,9 @@ Object.defineProperty(Signal.prototype, "value", {
  * @param value The initial value for the signal.
  * @returns A new signal.
  */
-function signal<T>(value: T): Signal<T> {
+export function signal<T>(value: T): Signal<T>;
+export function signal<T = undefined>(): Signal<T | undefined>;
+export function signal<T>(value?: T): Signal<T> {
 	return new Signal(value);
 }
 
@@ -821,4 +823,4 @@ function effect(fn: EffectFn): () => void {
 	return effect._dispose.bind(effect);
 }
 
-export { signal, computed, effect, batch, untracked, Signal, ReadonlySignal };
+export { computed, effect, batch, untracked, Signal, ReadonlySignal };

packages/preact/src/index.ts

@@ -345,8 +345,10 @@ Component.prototype.shouldComponentUpdate = function (
 	return false;
 };
 
-export function useSignal<T>(value: T) {
-	return useMemo(() => signal<T>(value), []);
+export function useSignal<T>(value: T): Signal<T>;
+export function useSignal<T = undefined>(): Signal<T | undefined>;
+export function useSignal<T>(value?: T) {
+	return useMemo(() => signal<T | undefined>(value), []);
 }
 
 export function useComputed<T>(compute: () => T) {

packages/react/runtime/src/index.ts

@@ -369,8 +369,10 @@ export function useSignals(usage?: EffectStoreUsage): EffectStore {
 	return _useSignalsImplementation(usage);
 }
 
-export function useSignal<T>(value: T): Signal<T> {
-	return useMemo(() => signal<T>(value), Empty);
+export function useSignal<T>(value: T): Signal<T>;
+export function useSignal<T = undefined>(): Signal<T | undefined>;
+export function useSignal<T>(value?: T) {
+	return useMemo(() => signal<T | undefined>(value), Empty);
 }
 
 export function useComputed<T>(compute: () => T): ReadonlySignal<T> {