Node JS Error: write after end - mysql

Question

In the following code, I am trying to retrieve data from MySQL database and show them to a user by using response write. The error that I got is Error: write after end:
var http = require("http");
var mysql = require('mysql');
var express = require('express');
var app = express();
var bodyParser = require('body-parser');
var urlencodedParser = bodyParser.urlencoded({ extended: false })
app.use(express.static('public'));
app.get('/Search.html', function (req, res) {
res.sendFile( __dirname + "/" + "Search.html" );
})
var connection = mysql.createConnection(
{
host : 'localhost',
user : 'root',
password : 'somepass',
database : 'SocialQuery',
}
);
connection.connect();
app.post('/process_post', urlencodedParser, function (req, res) {
// Prepare output in JSON format
response = {
SearchType:req.body.SearchTypes,
Term:req.body.term
};
//var vas = JSON.stringify(response);
var search = req.body.SearchTypes;
var term = req.body.term;
var query = connection.query('Select * from ?? where Lable = ?', [search, term], function(err, rows) {
res.write(rows);
});
console.log(query.sql);
res.end();
})
//}).listen(8081);
http.createServer(app).listen(8081);
console.log('Server running at http://127.0.0.1:8081/');
I changed res.write(rows); to res.end(rows); but didn't work. Can someone help me solving this problem.

Answer 1

The problem is that MySQL queries are asynchronous in node.js. so, the result won't be in the variable query, but retrieved in the callback, to the variable rows. So what happens is that res.end() is called, and then the callback returns and res.write() is called, so it's called after end().

Answer 2

You are doing an Asynchronous call when fetching data from database. res.write() is inside callback function so before fetching data it would call res.end() and res.write() will be called after the data has been fetched. That's why you are getting Error: write after end . You can use res.end() in the same callback function.
var query = connection.query('Select * from ?? where Lable = ?', [search, term], function(err, rows) {
res.write(rows, function(err){
res.end();
});
});
Now the res.end() function will be called after the write process has been done.

Answer 3

It worked after I made two changes:
var query = connection.query('Select * from ?? where Lable = ?', [search, term], function(err, rows) {
console.log(rows);
res.write(JSON.stringify(rows));
res.end();
});
First, I moved res.end(); inside the connection.query part.
Second, instead of writing rows only, I changed to res.write(JSON.stringify(rows));