useMiddle

// 中间件的使用
// 中间件是一个函数：三个参数： req, res, next
// 在这个函数中调用next()执行下一个中间件
// 中间件中的执行顺序： ABC CBA
var express = require('express');

var app = express();
app.listen(3000, function () {
    console.log('listen 3000...');
});

function middlewareA(req, res, next) {
    console.log('middlewareA before next()');
    next();
    console.log('middlewareA after next()');
}

function middlewareB(req, res, next) {
    console.log('middlewareB before next()');
    next();
    console.log('middlewareB after next()');
}

function middlewareC(req, res, next) {
    console.log('middlewareC before next()');
    next();
    console.log('middlewareC after next()');
}

app.use(middlewareA);
app.use(middlewareB);
app.use(middlewareC);

// middlewareA before next()
// middlewareB before next()
// middlewareC before next()
// middlewareC after next()
// middlewareB after next()
// middlewareA after next()