nodeJS zeroMQ: why cant send message if sock.send not in setInterval

I was using zeroMQ in nodeJS. But it seems that while sending the data from producer to worker, if I do not put it in setInterval, then it does not send the data to the worker. My example code is as follows:

producer.js
===========           
     var zmq = require('zmq')
      , sock = zmq.socket('push');

    sock.bindSync('tcp://127.0.0.1:3000');
    console.log('Producer bound to port 3000');
    //sock.send("hello");
    var i = 0;

     //1. var timer = setInterval(function() {
      var str = "hello";
      console.log('sending work', str, i++);

     sock.send(str);
     //2. clearTimeout(timer);
     //3. }, 150);

    sock.on('message', function(msg) {
    console.log("Got A message, [%s], [%s]", msg);
   });

So in the above code, if I add back the lines commented in 1, 2 and 3, then I do receive the message to the worker side, else it does not work.

Can anyone throw light why to send message I need to put it in setInterval? Or am I doing something wrong way?