Node's res.end() erases piped data

I'm new to node.js and just curious why res.end() erases my file

Let's say I have this code

index.js

var http = require('http');
var fs = require('fs');

http.createServer(function(req, res) {
    var file_to = fs.createWriteStream('file_to.txt');
    req.pipe(file_to);
    req.on('end', function() {
        res.end();
    });
}).listen(8080);

file_from.txt contains "copy me"

file_to.txt is empty

In terminal I do node index.js

In second terminal tab I do curl --upload-file file_from.txt http://localhost:8080

file_to.txt remains empty

If remove

req.on('end', function() {
    res.end();
});

file_to.txt will contain "copy me" as expected

Why?