how to disable res.writeHead() output extra number before output? - node.js

Question

I create a http server with express.
Below is the server code:
router.get('/', function(req, res, next) {
// req.socket.setTimeout(Infinity);
res.writeHead(200, {
'Content-Type': 'text/plain; charset=utf-8', // <- Important headers
'Cache-Control': 'no-cache',
'Connection': 'keep-alive',
});
res.write('hell');
res.write('world');
res.end();
// res.write('\n\n');
// response = res;
});
when I use netcat to GET the url.
the output is like this
GET /sse HTTP/1.1
HTTP/1.1 200 OK
X-Powered-By: Express
Content-Type: text/plain; charset=utf-8
Cache-Control: no-cache
Expires: 0
Connection: keep-alive
Keep-Alive: timeout=5, max=97
Date: Fri, 30 Jun 2017 11:50:00 GMT
Transfer-Encoding: chunked
4
hell
5
world
0
My question is why there are always a number before every res.write()? And the number seems is then length of the output of res.write.
How can I remove the number?

Answer 1

This is how the chunked encoding works. You don't have to declare up front how many bytes are you going to send but instead every chunk is prepended with the number of bytes that it has. For example: 4 for "hell", 5 for "world" and then finally 0 for "no more bytes to send".
You can see that you have the chunked encoding header present:
Transfer-Encoding: chunked
Now, to answer your question directly, to remove the numbers you'd have to switch off the chunked encoding. To do that you'd have to set the Content-length header to the number of bytes that you are going to send in the body of the response.
For example:
app.get('/', function(req, res, next) {
res.writeHead(200, {
'Content-Type': 'text/plain; charset=utf-8',
'Cache-Control': 'no-cache',
'Connection': 'keep-alive',
'Content-length': 9, // <<<--- NOTE HERE
});
res.write('hell');
res.write('world');
res.end();
});
(Of course in the real code you'd have to either calculate the length of the response or build up a string or buffer and get its length just before you set the Content-length header.)
But note that if you use curl or any other HTTP client then it will "remove" the numbers for you. It's just that with netcat you accidentally saw the underlying implementation detail of the chunked encoding but all of the real HTTP clients can handle that just fine.
Normally in HTTP you declare the length of the entire response in the headers and then send the body - in one piece, in multiple chunks, whatever - but it has to have the same length as what you declared. It means that you cannot start sending data before you know the length of everything that you want to send. With chunked encoding it's enough to declare the length of each chunk that you send but you don't have to know the length of the entire response - which could even be infinite, it's up to you. It lets you start sending as soon as you have anything to send which is very useful.