文档：http://expressjs.com/en/4x/api.html#router
一：app.get（）
// GET method route
app.get('/', function (req, res) {
res.send('GET request to the homepage')
})
单个回调函数可以处理路由。例如：
app.get('/example/a', function (req, res) {
res.send('Hello from A!')
})
多个回调函数可以处理路径（确保指定next对象）。例如：
app.get('/example/b', function (req, res, next) {
console.log('the response will be sent by the next function ...')
next()
}, function (req, res) {
res.send('Hello from B!')
})
二：app.post（）
// POST method route
app.post('/', function (req, res) {
res.send('POST request to the homepage')
})
三：app.all（）
app.all('/secret', function (req, res, next) {
console.log('Accessing the secret section ...')
next() // pass control to the next handler
})
四：app.route（）
app.route('/book')
.get(function (req, res) {
res.send('Get a random book')
})
.post(function (req, res) {
res.send('Add a book')
})
.put(function (req, res) {
res.send('Update the book')
})
五：express.Router
var express = require('express')
var router = express.Router()
// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
console.log('Time: ', Date.now())
next()
})
// define the home page route
router.get('/', function (req, res) {
res.send('Birds home page')
})
// define the about route
router.get('/about', function (req, res) {
res.send('About birds')
})
module.exports = router
var userRouter= require('./routes/index')
// ...
app.use('/user', userRouter)
