react: Add usePrevious.

Author: chrisbobbe

src/utils/react.js

@@ -0,0 +1,20 @@
+/* @flow strict-local */
+import { useRef, useEffect } from 'react';
+
+/**
+ * A Hook for the value of a prop, state, etc., from the previous
+ *   render, or the first render if this is the first.
+ *
+ * From
+ * https://reactjs.org/docs/hooks-faq.html#how-to-get-the-previous-props-or-state,
+ * which says, "It’s possible that in the future React will provide a
+ * `usePrevious` Hook out of the box since it’s a relatively common
+ * use case."
+ */
+export function usePrevious<T>(value: T): T {
+  const ref = useRef<T>(value);
+  useEffect(() => {
+    ref.current = value;
+  });
+  return ref.current;
+}