Tiny javascript spy for testing.

spy.js

function Spy (fn) {
  if (!fn) fn = function() {};

  function proxy() {
    var args = Array.prototype.slice.call(arguments);
    proxy.calls.push(args);
    proxy.called = true;
    fn.apply(this, args);
  }

  proxy.prototype = fn.prototype;
  proxy.calls = [];
  proxy.called = false;

  return proxy;
}

// Usage:
var event_spy = Spy(); 

// or, if you want a callback
var event_spy = Spy(function (data) {
  assert.equal(typeof data, 'object');
});

eventemitter.on('hello', event_spy);
eventemitter.emit('hello', { world: true });
eventemitter.emit('hello', { universe: 'better' });

// Result:
assert.ok(event_spy.called);
assert.equal(event_spy.calls.length, 2);