爲什麼？錯誤：發送後無法設置標題

Question

我不斷收到此錯誤：錯誤：發送後無法設置標題。

我已閱讀其他帖子，但我不明白。我沒有任何雙重回調或任何東西。我的代碼在哪裏導致這個錯誤？

 
Error: Can't set headers after they are sent. 
    at ServerResponse.OutgoingMessage.setHeader (http.js:689:11) 
    at ServerResponse.header (/root/node_modules/express/lib/response.js:666:10) 
    at ServerResponse.send (/root/node_modules/express/lib/response.js:146:12) 
    at fn (/root/node_modules/express/lib/response.js:900:10) 
    at View.exports.renderFile [as engine] (/root/node_modules/jade/lib/jade.js:330:12) 
    at View.render (/root/node_modules/express/lib/view.js:93:8) 
    at EventEmitter.app.render (/root/node_modules/express/lib/application.js:530:10) 
    at ServerResponse.res.render (/root/node_modules/express/lib/response.js:904:7) 
    at Query. (/root/tutsplus/server4.js:25:7) 
    at Query.emit (events.js:98:17) 

var express = require('express'), app = express(); 
var mysql = require('mysql'); 

app.get('/', function(req,res) { 

    var connection = mysql.createConnection({ 
     host: 'localhost', 
     user: 'root', 
     password: 'fuck', 
     database: 'store' 
    }); 

    var query = connection.query('SELECT * from category'); 

    query.on('result', function(row) { 

     var category = row.category_name; 

     res.render('fuck.jade', { 
      category: category 
     }); 
    }); 

}); // app.get 

app.listen(80, function() { 

    console.log('we are logged in'); 
}); 

Answer 1

正如我所說的在我的評論，這個問題幾乎總是被處理，這會導致響應件被稱爲無序異步操作不當造成的。

每一個使用.on()代碼示例here，您只需要結束請求時，您將獲得：

query.on('end', function() { 
    // all rows have been received 
}); 

我想你可能調用res.render()不止一次，因爲你在query.on('result', ...)調用它，而比收集完所有數據後的query.on('end', ....)。

，你在做這件事的事實：

query.on('result', ...) 

可能是導致問題的錯誤的計時問題。

---

從MySQL的NodeJS連接器文檔，這裏有一個例子：

var query = connection.query('SELECT * FROM posts'); 
query 
    .on('error', function(err) { 
    // Handle error, an 'end' event will be emitted after this as well 
    }) 
    .on('fields', function(fields) { 
    // the field packets for the rows to follow 
    }) 
    .on('result', function(row) { 
    // Pausing the connnection is useful if your processing involves I/O 
    connection.pause(); 

    processRow(row, function() { 
     connection.resume(); 
    }); 
    }) 
    .on('end', function() { 
    // all rows have been received 
    });