Link state with browser.storage.local via useEffect

popup/src/App.js

@@ -1,15 +1,73 @@
 /* global browser */
-import React, { useState } from 'react';
+import React, { useState, useEffect } from 'react';
 
 function App() {
-	const [ advanced, setAdvanced ] = useState(false);
-	const [ words, setWords ] = useState([]);
+	const [ advanced, setAdvanced ] = useState(undefined);
+	const [ words, setWords ] = useState(undefined);
 	const [ word, setWord ] = useState("");
-	const [ regex, setRegex ] = useState("");
-	const [ flags, setFlags ] = useState("");
+	const [ regex, setRegex ] = useState(undefined);
+	const [ flags, setFlags ] = useState(undefined);
 	function removeWord(idx) {
 		setWords([...words.slice(0, idx), ...words.slice(idx+1)]);
 	}
+	/*
+	 * Get inital state from storage.
+	 *
+	 * setterDefaultValue is key: [setter, defaultValue] object where each
+	 * setter sets the state while the defaultValue is set if the value is
+	 * undefined
+	 */
+	useEffect(() => {
+		const setterDefaultValue = {
+			advanced: [ setAdvanced, false ],
+			words: [ setWords, ["attendance", "roll number", "present"] ],
+			regex: [ setRegex, "" ],
+			flags: [ setFlags, "" ]
+		}
+		Object.keys(setterDefaultValue).forEach(key => {
+			const [ setter, defaultValue ] = setterDefaultValue[key];
+			browser.storage.local.get(key)
+				.then(value => {
+					if (key in value)
+						setter(value[key])
+					else {
+
+						browser.storage.local.set({ [key]: defaultValue });
+						setter(defaultValue);
+
+					}
+				})
+				.catch(() => {
+					browser.storage.local.set({ [key]: defaultValue });
+					setter(defaultValue);
+				});
+		});
+	}, []);
+
+	/* Sync state to storage
+	 *
+	 * For some reason browser.storage.local.set doesn't work unless I call it
+	 * from the callback of browser.storage.local.get. Don't touch.
+	 */
+	useEffect(() => {
+		browser.storage.local.get("regex")
+			.then((() => regex !== undefined && browser.storage.local.set({ regex: regex })));
+	}, [regex]);
+	useEffect(() => {
+		browser.storage.local.get("words")
+			.then((() => words !== undefined && browser.storage.local.set({ words: words })));
+	}, [words]);
+	useEffect(() => {
+		browser.storage.local.get("advanced")
+			.then((() => advanced !== undefined && browser.storage.local.set({ advanced: advanced })));
+	}, [advanced]);
+	useEffect(() => {
+		browser.storage.local.get("flags")
+			.then((() => flags !== undefined && browser.storage.local.set({ flags: flags })));
+	}, [flags]);
+	// Show Loading till all values are synced from storage to state
+	if ([advanced, words, regex, flags].map(i => i === undefined).reduce((i, j) => i || j))
+		return <div>Loading</div>;
 	return (
 		<>
 			<input