Store dark mode preference in localStorage

src/App.js

@@ -5,9 +5,10 @@ import { Switch, Route } from "react-router-dom";
 import PrivateRoute from "./components/PrivateRoute/";
 import AppView from "./components/AppView/";
 import LoginForm from "./components/LoginForm/";
+import { useStickyState } from "./utilities";
 
 function App() {
-	const [darkMode, setDarkMode] = useState(false);
+	const [darkMode, setDarkMode] = useStickyState(false);
 
 	const theme = webqueueTheme(darkMode);
 
@@ -20,7 +21,7 @@ function App() {
 				<PrivateRoute path="/">
 					<AppView setDarkMode={setDarkMode} />
 				</PrivateRoute>
-			</Switch>
+			</Switch>	
 		</ThemeProvider>
 	);
 }

src/utilities.js

@@ -1,3 +1,5 @@
+import { useState, useEffect } from "react";
+
 /**
  * Returns true if object has no entries, otherwise false.
  * @param {Object} objectToCheck The object to check.
@@ -7,4 +9,24 @@ function objectIsEmpty(objectToCheck) {
 	return Object.entries(objectToCheck).length === 0;
 }
 
-export { objectIsEmpty };
+/**
+ * Returns value of key in localStorage or default value in state. The state and localStorage value are updated together.
+ * @param {*} defaultValue The value to use if no value for key exist in localStorage.
+ * @param {string} key The key to lookup in localStorage.
+ * @returns {Array} Getter and setter of state variable.
+ * @see https://reactjs.org/docs/hooks-state.html
+ */
+function useStickyState(defaultValue, key) {
+	const [value, setValue] = useState(() => {
+		const stickyValue = window.localStorage.getItem(key);
+		return stickyValue !== null
+			? JSON.parse(stickyValue)
+			: defaultValue;
+	});
+	useEffect(() => {
+		window.localStorage.setItem(key, JSON.stringify(value));
+	}, [key, value]);
+	return [value, setValue];
+}
+
+export { objectIsEmpty, useStickyState };