on-new-listener.js

var e = new (require("events").EventEmitter)

e.on("newListener", function (ev, fn) {
  console.error(e.listeners(ev))
  console.error("During: listeners.length = %d", e.listeners(ev).length)
})

console.error("Before: listeners.length = %d", e.listeners("foo").length)
e.on("foo", function () {})
console.error(e.listeners("foo"))
console.error("After: listeners.length = %d", e.listeners("foo").length)

/*
Before: listeners.length = 0
[]
During: listeners.length = 0
[ [Function] ]
After: listeners.length = 1
*/

:)