中间件与路由处理器

路由处理器和中间件的参数中都有回调函数，这个函数有2个3个或4个参数。
如果有2个或3个参数，前两个参数是请求和响应对象，第三个参数是next函数。
如果有4个参数，它就变成了错误处理中间件，第一个参数变成了错误对象，然后依次是请求、响应和next对象。
如果不调用next()，管道就会被终止，也不会再有处理器或中间件做后续处理，此时应该发送一个响应到客户端。

var app = require('express')();
let i=1;
app.use(function(req, res, next){
    console.log('
ALLWAYS'+i++);
    next();
});
app.use(function(req, res, next){
    console.log('SOMETIMES');
    next();
});
//错误处理中间件
app.use(function(err, req, res, next){
    console.log('unhandled error detected: ' + err.message);
    res.send('500 - server error');
});
app.use(function(req, res){
    console.log('route not handled');
    res.send('404 - not found');
});
app.listen(3000, function(){
    console.log('listening on 3000');
});

var app = require('express')();
let i=1;
app.use(function(req, res, next){
      console.log('
ALLWAYS'+i++);
      next();
});
app.get('/a', function(req, res){
      console.log('/a: route terminated');
      res.send('a');
});
app.get('/a', function(req, res){
      console.log('/a: never called');
});
app.use(function(req, res, next){
      console.log('SOMETIMES');
      next();
});
//错误处理中间件
app.use(function(err, req, res, next){
      console.log('unhandled error detected: ' + err.message);
      res.send('500 - server error');
});
app.use(function(req, res){
      console.log('route not handled');
      res.send('404 - not found');
});
app.listen(3000, function(){
      console.log('listening on 3000');
});

var app = require('express')();
let i=1;
app.use(function(req, res, next){
      console.log('
ALLWAYS'+i++);
      next();
});
app.get('/b', function(req, res, next){
      console.log('/b: route not terminated');
      next();
});
app.use(function(req, res, next){
      console.log('SOMETIMES');
      next();
});
app.get('/b', function(req, res, next){
      console.log('/b (part 2): error thrown' );
      throw new Error('b failed');
});
app.use('/b', function(err, req, res, next){
      console.log('/b error detected and passed on');
      next(err);
});
//错误处理中间件
app.use(function(err, req, res, next){
      console.log('unhandled error detected: ' + err.message);
      res.send('500 - server error');
});
app.use(function(req, res){
      console.log('route not handled');
      res.send('404 - not found');
});
app.listen(3000, function(){
      console.log('listening on 3000');
});

 

var app = require('express')();
let i=1;
app.use(function(req, res, next){
    console.log('
ALLWAYS'+i++);
    next();
});
app.use(function(req, res, next){
    console.log('SOMETIMES');
    next();
});
app.get('/c', function(err, req){
    console.log('/c: error thrown');
    throw new Error('c failed');
});
app.use('/c', function(err, req, res, next){
    console.log('/c: error deteccted but not passed on');
    next();
});
//错误处理中间件
app.use(function(err, req, res, next){
    console.log('unhandled error detected: ' + err.message);
    res.send('500 - server error');
});
app.use(function(req, res){
    console.log('route not handled');
    res.send('404 - not found');
});
app.listen(3000, function(){
    console.log('listening on 3000');
});