Merge branch 'master' into fix/preactjs#2644

hooks/src/index.d.ts

@@ -50,7 +50,7 @@ type Ref<T> = { current: T };
  * Note that `useRef()` is useful for more than the `ref` attribute. It’s handy for keeping any mutable
  * value around similar to how you’d use instance fields in classes.
  */
-export function useRef<T>(initialValue: T | null): Ref<T>;
+export function useRef<T>(initialValue?: T | null): Ref<T>;
 
 /**
  * `useRef` without an initial value is the special case handling `ref` props.

hooks/src/index.js

@@ -174,7 +174,10 @@ export function useLayoutEffect(callback, args) {
 
 export function useRef(initialValue) {
 	currentHook = 5;
-	return useMemo(() => ({ current: initialValue }), []);
+	return useMemo(
+		() => ({ current: initialValue === undefined ? null : initialValue }),
+		[]
+	);
 }
 
 /**

hooks/test/browser/useRef.test.js

@@ -31,4 +31,20 @@ describe('useRef', () => {
 
 		expect(values).to.deep.equal([1, 2]);
 	});
+
+	it('defaults to null', () => {
+		const values = [];
+
+		function Comp() {
+			const ref = useRef();
+			values.push(ref.current);
+			ref.current = 2;
+			return null;
+		}
+
+		render(<Comp />, scratch);
+		render(<Comp />, scratch);
+
+		expect(values).to.deep.equal([null, 2]);
+	});
 });