Listeners.js

var EventEmitter = require("events").EventEmitter;

var emitter = new EventEmitter();

emitter.once("foo", function() {
  console.log("foo");
});

emitter.on("foo", function() {
  console.log("bar");
});

var listeners = emitter.listeners("foo"); // => [function(){} bound with "once", function(){} bound with "on"]

listeners.length; // => 2
listeners[0](); // Manually calling the listener that was bound with "once" removes it from the array
listeners.length; // => 1