Node.js TCP client behaving differently from Netcat/Telnet

Question

Node.js TCP client behaving differently from Netcat/Telnet

I am issuing newline-separated text commands to a custom protocol TCP server. In the example below I issue 2 commands and receive a response written back. It works as expected in telnet and netcat:

$ nc localhost 1234
command1
command2
theresponse

The same workflow is not working when connecting with Node.js:

var net = require('net');
var client = net.connect(1234, 'localhost');

client.on('data', function(data) {
    console.log('data:', data.toString());
});

client.on('error', function(err) {
    console.log('error:', err.message);
});

client.write('command1\n');
client.write('command2\n');

I would expect that after running this program I would see "data: theresponse" written to the console, however, nothing is ever printed. I have also tried performing the writes inside of the "connect" callback, but I have the same results. The curious thing is that when I try this in the Node REPL...it works:

$ node
> var net = require('net')
undefined
> var client = net.connect(1234, 'localhost')
undefined
> client.on('data', function(data) { console.log('data:', data.toString()); })
{ ... }
> client.write('command1\n')
true
> client.write('command2\n')
true
> data: theresponse

Anyone have ideas about this bizarre behavior?

Thanks.

-Scott

sockets
node.js
tcp
telnet
netcat

Answer 1

Without testing the code, I'm presuming it's the asynchronous nature of Node.js that's biting you. In the REPL the connection happens before you can type in another command. In your code above you are writing before you are connecting.

Change the above code to this:

var net = require('net');
var client = net.connect(1234, function(){
   client.on('data', function(data) {
     console.log('data:', data.toString());
   });

   client.on('error', function(err) {
     console.log('error:', err.message);
   });

   client.write('command1\n');
   client.write('command2\n');
});