Difference b/w event.on() and event.once() in nodejs

Question

I'm testing the plus_one app, and while running it, I just wanted to clarify my concepts on event.once() and event.on()

This is plus_one.js

> process.stdin.resume();
process.stdin.on('data',function(data){
    var number;
    try{
        number=parseInt(data.toString(),10);
        number+=1;
        process.stdout.write(number+"\n");
        }
    catch(err){
        process.stderr.write(err.message+"\n");
        }
    });

and this is test_plus_one.js

var spawn=require('child_process').spawn;
var child=spawn('node',['plus_one.js']);

setInterval(function(){
    var number=Math.floor(Math.random()*10000);
    child.stdin.write(number+"\n");
    child.stdout.on('data',function(data){
        console.log('child replied to '+number+' with '+data);
        });
    },1000);

I get few maxlistener offset warning while using child.stdin.on() but this is not the case when using child.stdin.once(), why is this happening ?

Is it because child.stdin is listening to previous inputs ? but in that case maxlistener offset should be set more frequently, but its only happening for once or twice in a minute.

Plantaineater

Answer 1

Using EventEmitter.on(), you attach a full listener, versus when you use EventEmitter.once(), it is a one time listener that will detach after firing once. Listeners that only fire once don't count towards the max listener count.

Lionize

Answer 2

According to the latest official docs https://nodejs.org/api/events.html#events_eventemitter_defaultmaxlisteners. The .once() listener does count to the maxlisteners.

emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

Bioscopy