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.

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.

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)); });