Implement state in URL

src/App.tsx

@@ -1,16 +1,78 @@
-import React, {useState} from 'react';
+import React from 'react';
 import './App.css';
 import {Calendar} from './Calendar';
 import {gregorianJDN, Month} from "./dates";
 
-function App() {
+type YearMonth = {
+    year: number;
+    month: Month;
+}
+
+function parseURL(): YearMonth | null {
+    const match = /\/(-?\d+)\/(\d+)/.exec(window.location.pathname);
+    if (!match)
+        return null;
+
+    const month = +match[2];
+    if (month < 1 || month > 13)
+        return null;
+    return {year: +match[1], month: month as Month};
+}
+
+type AppState = YearMonth & {
+    todayJDN: number,
+};
+
+class App extends React.Component<{}, AppState> {
+    state: AppState;
+
+    constructor(props: {}) {
+        super(props);
+        const current = {year: 230, month: 5};
         const today = new Date();
-    const todayJDN = gregorianJDN(today.getFullYear(), today.getMonth() + 1, today.getDay());
-    const [yearMonth, setYearMonth] = useState([230, 5]);
-    return (
-        <Calendar year={yearMonth[0]} month={yearMonth[1] as Month} todayJDN={todayJDN}
-                  onSwitch={(year, month) => setYearMonth([year, month])}/>
-    );
+
+        this.state = {
+            ...(parseURL() || current as YearMonth),
+            todayJDN: gregorianJDN(today.getFullYear(), today.getMonth() + 1, today.getDay()),
+        };
+        this.updateURL();
+        this.updateStateFromURL = this.updateStateFromURL.bind(this);
+    }
+
+    componentDidMount() {
+        window.addEventListener('popstate', this.updateStateFromURL);
+    }
+
+    componentWillUnmount() {
+        window.removeEventListener('popstate', this.updateStateFromURL);
+    }
+
+    private updateStateFromURL(event: PopStateEvent) {
+        this.setState(event.state);
+    }
+
+    private updateURL() {
+        const {year, month} = this.state;
+        const path = `/${year}/${month}`;
+        if (path !== window.location.pathname) {
+            window.history.pushState({year, month}, '', path);
+        }
+    }
+
+    setState(state: any, callback?: () => void) {
+        super.setState(state, () => {
+            this.updateURL();
+            callback && callback();
+        });
+    }
+
+    render() {
+        return <Calendar
+            year={this.state.year} month={this.state.month} todayJDN={this.state.todayJDN}
+            onSwitch={(year, month) => {
+                this.setState({year, month})
+            }}/>;
+    }
 }
 
 export default App;