当我取消注释一行代码时，我收到此错误： _http_outgoing.js:359 throw new Error('Can\t set headers after they are sent.');

当我取消注释这一行时：return done(null, false, { message: 'Incorrect username' });在下面的代码中，Node.js 运行没有任何错误，否则 Node.js 会给出后面提到的错误：

//we need to define a local Strategy to authenticate with username and password
passport.use(new LocalStrategy(
    function(username, password, done) { //start of custom code
        //from here starts our custom code (inside this function scope)
        //outside this scope is from passport API documentation
        //process.nextTick(function() {
        UserFind(username, function(err, user) {
                if (err) {
                    return done(err);
                } else if (!user || user.length == 0) {
                    //When I uncomment the following line of code, an error occurs, but when I comment out the following, no error is received:
                    //return done(null, false, { message: 'Incorrect username' });
                } else {
                    console.log('user._id:' + user._id);
                    console.log('typeof user: ' + typeof(user));
                    console.log('user.username: ' + user.username);
                    console.log('user.password: ' + user.password);
                    if (password !== user.password) {
                        return done(null, false, { message: 'Incorrect password' });
                    }
                    return done(null, user);
                }

            })
            //}) //process.nextTick
    } //end of custom code
));

当我取消注释上述行时收到的错误：

_http_outgoing.js：359 throw new Error('发送后无法设置标头。'); ^

错误：发送后无法设置标头。 在 ServerResponse.setHeader (_http_outgoing.js:359:11) 在 ServerResponse.header (/home/ict/Documents/hookahDB/serverjs/node_modules/express/lib/response.js:719:10) 在 ServerResponse.location (/home/ict/Documents/hookahDB/serverjs/node_modules/express/lib/response.js:836:15) 在 ServerResponse.redirect (/home/ict/Documents/hookahDB/serverjs/node_modules/express/lib/response.js:874:18) 完全失败（/home/ict/Documents/hookahDB/serverjs/node_modules/passport/lib/middleware/authenticate.js:132:20） 尝试时（/home/ict/Documents/hookahDB/serverjs/node_modules/passport/lib/middleware/authenticate.js:167:28） 在Strategy.strategy.fail（/home/ict/Documents/hookahDB/serverjs/node_modules/passport/lib/middleware/authenticate.js:284:9） 已验证（/home/ict/Documents/hookahDB/serverjs/node_modules/passport-local/lib/strategy.js:82:30） 在/home/ict/Documents/hookahDB/serverjs/index.js:158:28 在/home/ict/Documents/hookahDB/serverjs/index.js:144:16

我想知道为什么这一行是导致错误的原因以及如何解决它。谢谢。

EDIT

回调函数如下所示：

function UserFind(username, cb) {
    db.view('users/by_username', function(err, res) {
        if (err) {
            //db.view returned error
            return cb(err);
        }
        res.forEach(function(key, value, id) {
                //1st input=key|username, 2nd input=value|userDocument, 3rd input=id|_id
                //console.log('key: '+key+' row: '+row+' id: '+ id);
                if (username === key) {
                    //found the user
                    return cb(false, value);
                }
            })
            //couldn't find the user by forEach loop
        return cb(false, false);
    })
}

该错误消息是由处理异步响应时的计时错误引起的，该错误导致您在发送响应后尝试发送响应上的数据。

当人们将快速路由内的异步响应视为同步响应并且最终发送数据两次时，通常会发生这种情况。

您应该在声明中添加 else

if (password !== user.password) {
   return done(null, false, { message: 'Incorrect password' });
} else {
    return done(null, user);
}

Update:

function UserFind(username, cb) {
    var userFound = false, userVal = "";
    db.view('users/by_username', function(err, res) {
        if (err) {
            //db.view returned error
            return cb(err);
        }
        res.forEach(function(key, value, id) {
                //1st input=key|username, 2nd input=value|userDocument, 3rd input=id|_id
                //console.log('key: '+key+' row: '+row+' id: '+ id);
                if (username === key) {
                    //found the user
                    userFound = true;
                    userVal = value;
                }
            });

       if (userFound) {
          return cb(false, userVal);
       } else {
         // User did not found
         return cb(false, false);
       }
    })
}