we have persistence!

server/index.js

@@ -1,4 +1,5 @@
 // @ts-check
+const fs = require('fs');
 const express = require('express');
 const bodyParser = require('body-parser');
 const cors = require('cors');
@@ -6,6 +7,20 @@ const app = express();
 
 const store = {
   /** @type {any} */
+  read() {
+    if (fs.existsSync('tmp.json')) {
+      store.todos = JSON.parse(fs.readFileSync('tmp.json').toString());
+    } else {
+      store.todos = {};
+    }
+
+    return store.todos;
+  },
+
+  save() {
+    fs.writeFileSync('tmp.json', JSON.stringify(store.todos));
+  },
+
   todos: {}
 };
 
@@ -13,24 +28,28 @@ app.use(bodyParser.json());
 app.use(cors());
 
 app.get('/todos', (req, res) => {
-  res.json(store.todos);
+  res.json(store.read());
 });
 
 app.put('/todos/:id', (req, res) => {
   store.todos[req.params.id] = req.body;
+  store.save();
   res.json('ok');
 });
 
 app.post('/todos/:id', (req, res) => {
   store.todos[req.params.id] = req.body;
+  store.save();
 });
 
 app.delete('/todos/:id', (req, res) => {
   delete store.todos[req.params.id];
+  store.save();
 });
 
 app.post('/todos', req => {
   store.todos = req.body;
+  store.save();
 });
 
 app.get('/hello', (req, res) => {