Node.js，Express：将标头发送到客户端后无法设置标头

Question

首先，我阅读了所有其他StackOverflow答案和GitHub问题，但似乎都没有解决我的问题。使用以下代码（不建议使用其中的一些，脏代码表示抱歉）：

var express = require('express');
var fs = require('fs');
var path = require('path');
var parser = require('body-parser');
//Initializing the Express Framework
const app = express();
const mongoose = require('mongoose');
const uuidv4 = require ('uuid/v4');
var jose = require('node-jose');
var secureRandom = require('secure-random');
var bcrypt = require('bcrypt');
const jwt = require('express-jwt');
const jwt2 = require('jsonwebtoken');
var userSchema = new mongoose.Schema(
  username: type:String,unique:true,
  password: String
,timestamps:true)

var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://localhost:27017/";

var signingKey = secureRandom(256, type: 'Buffer');

app.use(express.json());
app.use(express.urlencoded( extended: true ));

// Add headers
app.use(function (req, res, next) 
  res.removeHeader('X-Powered-By');
  res.setHeader('Access-Control-Allow-Origin', 'http://localhost:4200');
  res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');
  res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type,Authorization');
  res.setHeader('Access-Control-Allow-Credentials', true);
  next();
);

var claims = 
  iss: "http://example.com/",  // The URL of your service
  sub: "users/admin1",    // The UID of the user in your system
  name: "User Name",
  scope: "self, admins"


userSchema.pre('save', function (next) 
  var user = this;
  if (!user.isModified('password')) return next();
  bcrypt.hash(user.password,10).then((hashedPassword) => 
      user.password = hashedPassword;
      next();
  )
, function (err) 
  next(err);
)
userSchema.methods.comparePassword=function(candidatePassword,next)    bcrypt.compare(candidatePassword,this.password,function(err,isMatch)
      if(err) return next(err);
      next(null,isMatch)
  )

module.exports = mongoose.model("user", userSchema);

function protectRoute(req,res,next)
  // if user exists the token was sent with the request
  if(req.user)
   //if user exists then go to next middleware
     next();
  
// token was not sent with request send error to user
  else
     res.status(500).json(error:'login is required');
  


/* app.get('/protected',
  jwt(secret: 'shhhhhhared-secret'),
  function(req, res) 
    if (!req.user.admin) return res.sendStatus(401);
    res.sendStatus(200);
  ); */

MongoClient.connect(url,  
  useUnifiedTopology: true,
  useNewUrlParser: true,
  ,function(err, db) 
  if (err) throw err;
  var dbo = db.db("demo");
  dbo.collection("demo").find().toArray(function(err, result) 
    if (err) throw err;
    console.log(result);
    db.close();
  );
); 

readJsonFile = function(fname, res) 

    fs.readFile(fname, 'utf8', function (err,data) 
        if (err) 
            console.log(err);
        
        res.send(data.toString().replace(/\n|\r/g,''));
    );




app.get('/', function (req, res) 
  res.send('Hello World!');
);

app.get('/users', function (req, res) 
    MongoClient.connect(url,  
    useUnifiedTopology: true,
    useNewUrlParser: true,
    ,function(err, db) 
    if (err) throw err;
    var dbo = db.db("demo");
    dbo.collection("users").find().toArray(function(err, result) 
      if (err) throw err;
      res.send(result);
      db.close();
    );
  ); 
  console.log("GET UserCollection from MongoDB");
);

app.post('/users/:id', verifyToken, function (req, res)
  jwt2.verify(req.token, 'secretkey', (err, auth) =>
    if(err)
      res.sendStatus(403);

     else 
      res.json(
        message: 'User created (in theory)',
        authData
      )
    
  )
  res.json(
    message: 'User created (in theory)2'
  )
);

app.post('/api/login', function (req, res)
  // Mock user TODO: implement proper BackEnd!
  const user = 
    id:1,
    username:'brad',
    email: 'brad@gmail.com'
  
  jwt2.sign(user, 'secretkey',  expiresIn: '1h', (err, token) =>
    res.json(
      token
    );
  );
);

app.get('/user', function (req, res) 
    readJsonFile('user.json', res);
  console.log("GET User");
);

// FORMAT OF TOKEN
// Authorization: Bearer <access_token>

// Verify Token
function verifyToken(req, res, next)
  // Get auth header value
  const bearerHeader = req.headers['authorization'];
  // Check if bearer is undefined
  if(typeof bearerHeader !== 'undefined')
    // Split at the space
    const bearer = bearerHeader.split(' ');
    // Get token from array
    const bearerToken = bearer[1];
    // Set the token
    req.token = bearerToken;
    // Next middleware
    next();
    res.json(message: "Login successful");
  
    else
      // Forbidden
      res.sendStatus(403);
      next();
    
  

app.listen(3000, function () 
  console.log('Listening on port 3000!');
);

if(process.env.NODE_ENV !== 'production') 
  process.once('uncaughtException', function(err) 
    console.error('FATAL: Uncaught exception.');
    console.error(err.stack||err);
    setTimeout(function()
      process.exit(1);
    , 100);
  );

运行应用程序并发布URL / users / 5后，出现以下错误，应用程序崩溃：

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:485:11)
    at Array.write (C:\Users\User\Documents\Project\Project2\node_modules\finalhandler\index.js:285:9)
    at listener (C:\Users\User\Documents\Project\Project2\node_modules\on-finished\index.js:169:15)
    at onFinish (C:\Users\User\Documents\Project\Project2\node_modules\on-finished\index.js:100:5)
    at callback (C:\Users\User\Documents\Project\Project2\node_modules\ee-first\index.js:55:10)
    at IncomingMessage.onevent (C:\Users\User\Documents\Project\Project2\node_modules\ee-first\index.js:93:5)
    at IncomingMessage.emit (events.js:215:7)
    at endReadableNT (_stream_readable.js:1183:12)
    at processTicksAndRejections (internal/process/task_queues.js:80:21)
ReferenceError: authData is not defined
    at C:\Users\User\Documents\Project\Project2\src\app.js:134:9
    at C:\Users\User\Documents\Project\Project2\node_modules\jsonwebtoken\verify.js:223:12
    at getSecret (C:\Users\User\Documents\Project\Project2\node_modules\jsonwebtoken\verify.js:90:14)
    at Object.module.exports [as verify] (C:\Users\User\Documents\Project\Project2\node_modules\jsonwebtoken\verify.js:94:10)       
    at C:\Users\User\Documents\Project\Project2\src\app.js:127:8
    at Layer.handle [as handle_request] (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\layer.js:95:5)    
    at next (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\route.js:137:13)
    at verifyToken (C:\Users\User\Documents\Project\Project2\src\app.js:178:5)
    at Layer.handle [as handle_request] (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\layer.js:95:5)    
    at next (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\layer.js:95:5)    
    at C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\index.js:281:22
    at param (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\index.js:354:14)
    at param (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\index.js:365:14)
    at Function.process_params (C:\Users\User\Documents\Project\Project2\node_modules\express\lib\router\index.js:410:3)

我尝试添加return和next语句无济于事。

P.S这有助于理解问题：

此错误发生在您之前发送响应，然后尝试再次发送响应时。为此，您必须检查是否有任何一段代码发送了两次响应。有时由于nodejs的异步行为而发生。有时，一个流程将进入事件循环，我们发送响应，完成后，将再次发送响应。因此，您可以使用回调或异步等待来等待执行。

Answer 1

另一答案