普通middleware

const express = require('express')
const app = express()
const port = 3000

app.use('/normal', (req, res) => res.send('Hello World!'))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

文艺middleware (async)

const express = require('express')
const app = express()
const port = 3000

app.use('/art', async (req, res) => {
  await new Promise((resolve, reject) => {
    setTimeout(resolve, 1000)
  })
  res.send('Bonjour le monde!')
})

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

万一middleware里报错了怎么办

增加一个4参数的中间件作为error handler，当然express也有一个默认的handler

const express = require('express')
const app = express()
const port = 3000

app.use('/normal', (req, res) => res.send('Hello World!'))
app.use('/normal-error', (req, res, next) => {
  throw new Error('some error')
})

app.use((err, req, res, next) => res.send('error detected: ' +  err.message))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

async middleware里的报错也这么处理？

答案是No

const express = require('express')
const app = express()
const port = 3000

app.use('/art', async (req, res) => {
  await new Promise((resolve, reject) => {
    setTimeout(resolve, 1000)
  })
  res.send('Bonjour le monde!')
})
app.use('/2b-error-1', async (req, res) => {
  await new Promise((resolve, reject) => {
    setTimeout(() => reject('another error 1'), 1000)
  })
})
app.use('/2b-error-2', async (req, res) => {
  await Promise.resolve().then(() => {
    throw new Error('another error 2')
  })
})

app.use((err, req, res, next) => res.send('error detected: ' +  err.message))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

如果只这样写的话，浏览器里不会有任何返回，而你会在终端里得到下面的报错：

正确处理async middleware里的报错

const express = require('express')
const app = express()
const port = 3000

app.use('/art', async (req, res) => {
  await new Promise((resolve, reject) => {
    setTimeout(resolve, 1000)
  })
  res.send('Bonjour le monde!')
})
app.use('/art-error-1', async (req, res, next) => {
  try {
    await new Promise((resolve, reject) => {
      setTimeout(() => reject(new Error('another error 1')), 1000)
    })
  } catch (e) {
    next(e)
  }
})
app.use('/art-error-2', async (req, res, next) => {
  await Promise.resolve().then(() => {
    throw new Error('another error 2')
  }).catch(next)
})

app.use((err, req, res, next) => res.send('error detected: ' +  err.message))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

处理的办法就是catch住异步操作中可能会抛出的错误，这里其实没有新的内容，在express的文档里已经说了必须捕获异步操作中的错误：

You must catch errors that occur in asynchronous code invoked by route handlers or middleware and pass them to Express for processing.

每个异步操作都如此try/catch，太麻烦了

解决办法是写一个高阶函数asyncMiddleware，在这个函数里面给传进来的middleware自动加上catch方法。

const express = require('express')
const app = express()
const port = 3000

app.use('/art', async (req, res) => {
  await new Promise((resolve, reject) => {
    setTimeout(resolve, 1000)
  })
  res.send('Bonjour le monde!')
})

// 重点
function asyncMiddleware(fn) {
  return (req, res, next) => fn(req, res, next).catch(next)
}
app.use('/art-error-1', asyncMiddleware(async (req, res, next) => {
  await new Promise((resolve, reject) => {
    setTimeout(() => reject(new Error('another error 1')), 1000)
  })
}))
app.use('/art-error-2', asyncMiddleware(async (req, res, next) => {
  await Promise.resolve().then(() => {
    throw new Error('another error 2')
  })
}))

app.use((err, req, res, next) => res.send('error detected: ' +  err.message))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

如果不确定参数是否一定为async function，可以使用Promise.resolve(fn(req, res, next))