[Gordon] add one example for async/await with express

Author: songguoqiang

async_await_example.js

@@ -0,0 +1,48 @@
+const express = require("express");
+const app = express();
+
+const wrapAsync = fn => (req, res, next) => {
+  Promise.resolve(fn(req, res, next)).catch(err => next(err));
+};
+
+// this function simulates one asynchronous operation,
+// e.g. loading user profile from database
+const loadUserProfileFromDB = userName => {
+  return new Promise(resolve => {
+    setTimeout(() => resolve({ name: userName, gender: "M" }), 10);
+  });
+};
+
+const getUserProfile = async (req, res, next) => {
+  const userName = req.params.userName;
+  const userProfile = await loadUserProfileFromDB(userName);
+  res.send(userProfile);
+};
+
+app.get("/users/:userName", wrapAsync(getUserProfile));
+
+// this function simulates one asynchronous operation that generate errors,
+// e.g. loading an blog entry from database
+const loadBlogPostFromDB = postId => {
+  return new Promise((resolve, reject) => {
+    setTimeout(() => reject(new Error("Network Connection Error")), 10);
+  });
+};
+
+const getBlogPost = async (req, res, next) => {
+  const postId = req.params.postId;
+  // note: this line below would throw error
+  const post = await loadBlogPostFromDB(postId);
+  res.return(post);
+};
+
+app.get("/posts/:postId", wrapAsync(getBlogPost));
+
+app.use(function(err, req, res, next) {
+  res.status(500);
+  res.send({ err: err.message });
+});
+
+const server = app.listen(3000, function() {
+  console.log("Application started....");
+});