add code

4/express/myapp-async/app.js

@@ -0,0 +1,51 @@
+const express = require('express')
+const app = express()
+const port = 3000
+
+app.listen(port, () => console.log(`Example app listening on port ${port}!`))
+/**
+ * 中间件 1
+ */
+app.use(async (req, res, next) => {
+    console.log('first');
+    await next();
+    console.log('first end');
+});
+/**
+ * 中间件 2
+ */
+app.use(async (req, res, next) => {
+    console.log('second');
+    await next();
+    console.log('second end');
+});
+
+/**
+ * 异步中间件
+ */
+app.use(async (req, res, next) => {
+    console.log('async');
+    await next();
+    await new Promise(
+        (resolve) => 
+            setTimeout(
+                () => {
+                    console.log(`wait ${s} ms end`);
+            resolve()
+        }, 
+      1000)
+    );
+    console.log('async end');
+});
+
+/**
+ * 中间件 3
+ */
+app.use(async (req, res, next) => {
+    console.log('third');
+    await next();
+    console.log('third end');
+});
+
+app.get('/', (req, res) => res.send('Hello World!'))
+