store locale into localstorage

src/i18n/index.tsx

@@ -4,6 +4,9 @@ import React, {
 import fi from "./locales/fi/common.json";
 import en from "./locales/en/common.json";
 
+type Lang = "fi" | "en";
+const LOCAL_STORAGE_KEY = "locale";
+
 type TranslateFunc = (key: string) => string;
 
 const translateEn: TranslateFunc = (key) => {
@@ -24,16 +27,24 @@ const translateFi: TranslateFunc = (key) => {
 };
 
 interface Store {
-  language: string;
-  changeLanguage: React.Dispatch<string>,
+  language: Lang;
+  changeLanguage: React.Dispatch<Lang>,
+}
+
+let initialLanguage: Lang = "fi";
+try {
+  const storedLang = localStorage.getItem(LOCAL_STORAGE_KEY) as Lang;
+  initialLanguage = storedLang;
+} catch (err) {
+  // Just ignore if fails to get value from browser (server etc.)
 }
 
 const initialState: Store = {
-  language: "fi",
+  language: initialLanguage,
   changeLanguage: null,
 };
 
-const Reducer = (state: Store, action: string) => {
+const Reducer = (state: Store, action: Lang) => {
   switch (action) {
     case "fi":
       return {
@@ -53,8 +64,15 @@ const Reducer = (state: Store, action: string) => {
 const LocaleContext = createContext(initialState);
 
 const LocaleStore: React.FC = ({ children }) => {
-  const [state, changeLanguage] = useReducer(Reducer, initialState);
-
+  const [state, dispatch] = useReducer(Reducer, initialState);
+  const changeLanguage = (action: Lang) => {
+    dispatch(action);
+    try {
+      localStorage.setItem(LOCAL_STORAGE_KEY, action);
+    } catch (err) {
+      // Just ignore if fails to store value in user's browser
+    }
+  };
   return (
     <LocaleContext.Provider value={{ ...state, changeLanguage }}>
       {children}