express的next()函数

express的next()函数

在定义middleware和route的时候我们经常看到next函数，表示跳转到下一个middle或者route函数，这是最常见的用法，其next()是不带参数的；其实next()函数还可以带参数，总结如下：

1. next() 不带参数
   表示继续跳转执行下一个middle或者route函数。
2. next('route')
   表示丢弃当前route的剩余callback函数，注意只在由app.METHOD()/route.METHOD()中定义的middleware函数中生效。
3. next(! 'route')
   对于任何非'route'参数(不管是一个简单数字或者还是一个字符串)都被认为是一个err，跳转到错误处理函数。

这儿我们举几个例子说明next('route')的用法。

app.get('/hello', (req, res, next) => {
  console.log('in get 1');
  next()
}, (req, res, next) => {
  console.log('in get 2');
  next()
}, (req, res, next) => {
  console.log('in get 3');
  next()
});

app.get('/hello', (req, res, next) => {
  console.log('in get 4');
});

在这个例子中四个callback都会被调用到。
我们改一下在第二个callback中使用next('route')

app.get('/hello', (req, res, next) => {
  console.log('in get 1');
  next()
}, (req, res, next) => {
  console.log('in get 2');
  next('route')  // this will skip the next callback
}, (req, res, next) => {
  console.log('in get 3');
  next()
});

app.get('/hello', (req, res, next) => {
  console.log('in get 4');
});

运行结果呢：

in get 1
in get 2
in get 4

可见第三个callback函数被skip掉了。

如果我们定义router.get()函数也是这个结果：

const express = require('express');
const app = express();
const router = express.Router()

router.get('/hello', (req, res, next) => {
  console.log('in get 1');
  next()
}, (req, res, next) => {
  console.log('in get 2');
  next()
}, (req, res, next) => {
  console.log('in get 3');
  next()
});

router.get('/hello', (req, res, next) => {
  console.log('in get 4');
});

app.use('/', router);

此时四个callback都能被回调。

const express = require('express');
const app = express();
const router = express.Router()

router.get('/hello', (req, res, next) => {
  console.log('in get 1');
  next()
}, (req, res, next) => {
  console.log('in get 2');
  next('route')  // this will skip the next callback
}, (req, res, next) => {
  console.log('in get 3');
  next()
});

router.get('/hello', (req, res, next) => {
  console.log('in get 4');
});

app.use('/', router);

此时只有三个callback能够被回调，即：

in get 1
in get 2
in get 4