Listeners.js

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

var emitter = new EventEmitter();

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

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

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

// Manually calling the handlers themselves doesn't change their event binding
foo();
bar();

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