spy.js

function spy() {
  var called = false
    , args = null

  function spyFn() {
    called = true
    args = Array.prototype.slice.call(arguments)
  }

  spyFn.calledWith = function calledWith() {
    return args
  }

  spyFn.reset = function reset() {
    called = false
    args = null
  }

  return spyFn
}