错误 [ERR_HTTP_HEADERS_SENT]: 发送到客户端后无法设置标头 2答案

【问题标题】：Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client 2错误 [ERR_HTTP_HEADERS_SENT]: 发送到客户端后无法设置标头 2
【发布时间】：2021-12-07 02:44:46
【问题描述】：

这是我的代码最初的样子：

const express = require('express')
const app = express()
const morgan = require('morgan')

app.use(morgan('tiny'));
// app.use((req, res, next) => {
//     console.log("OUR MIDDLEWARE")
//     console.log(req.method.toUpperCase(), req.path);
//     next();
// })

app.use((req, res, next) => {
    console.log(req.query);
    const { pwd } = req.query

    if (pwd === 'chickennugget') {
        next();
    }
    res.send("Sorry, you need a password")
})

app.get('/', (req, res) => {
    res.send("HOME PAGE!!")
})

app.get('/dogs', (req, res) => {
    res.send("WOOF  WOOF!!")
})



app.listen(3000, () => {
    console.log("App is running on localhost:3000")
})

它产生了这个错误：

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:371:5)
    at ServerResponse.setHeader (node:_http_outgoing:576:11)
    at ServerResponse.header (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/response.js:771:10)
    at ServerResponse.send (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/response.js:170:12)
    at /Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/app.js:19:9
    at Layer.handle [as handle_request] (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/router/layer.js:95:5)
    at trim_prefix (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/router/index.js:317:13)
    at /Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/router/index.js:284:7
    at Function.process_params (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/router/index.js:335:12)
    at next (/Users/tebahsaboun/Desktop/STUDIES/UDEMY/TWDB 2021/Section 40: Express/Middleware_Intro/node_modules/express/lib/router/index.js:275:10)

然后我本能地通过修改这段特定的代码来修复它：

app.use((req, res, next) => {
    console.log(req.query);
    const { pwd } = req.query

    if (pwd === 'chickennugget') {
        return next();
    }
    res.send("Sorry, you need a password")
})

（我在 next() 函数调用之前引入了一个 return。）

我的问题有两个：

为什么初始代码会出现此错误？
为什么要引入“return”关键字。修复错误？

我浏览了 StackOverFlow 以获得答案，并且有一个关于此错误的现有线程，但我看不出答案如何适用于我的特定情况。

非常感谢您的帮助。

最好的，

【问题讨论】：

标签： express middleware

【解决方案1】：

在初始代码中，错误是因为在没有return 语句的情况下，服务器尝试发送两个响应。第一个来自中间件：

res.send("Sorry, you need a password")

第二个是来自实际路线的响应。

return 修复了错误，因为当变量 pwd 等于 "chickennugget" 时，return 语句将代码停止在那里并且从不读取：

res.send("Sorry, you need a password")

You can read about the return statement here.

【讨论】：

谢谢@monge1h