Callback is being called only after big for loop ends

I'm receiving data in browser through websockets (paho-mqtt) but problem is that the receiving callback gets fired only when another task ends (big for loop) and it gets fired with all the stacked data, I'm not losing data just getting delayed. Shouldn't the callback get fired even if there is a loop running? What is happening here?. Otherwise, how can I achieve this, keep receiving while inside a loop?

What I'm trying to say is equivalent to the following:

If I do this in chrome

setTimeout(() => {
  console.log('hello!');
}, 10);
for (var i = 0; i < 50000; i++) {
  console.log('for array');
}

I get

50000 VM15292:5 for array
VM15292:2 hello!

Shouldn't I get something like this?

1000 VM15292:5 for array
VM15292:2 hello!
49000 VM15292:5 for array