oncall.js

var EventEmitter = require('events').EventEmitter

// my current implementation

module.exports = function (obj) {
  var calls = new EventEmitter()
  for(methodName in obj) {
    if (typeof obj[methodName] !== 'function') continue

    var originalMethod = obj[methodName]
    obj[methodName] = (function (methodName, originalMethod) {
      return function() {
        calls.emit
          .bind(calls, methodName)
          .apply(calls, arguments)
        return originalMethod.apply(obj, arguments)
      }
    })(methodName, originalMethod)
  }
  return calls
}

spy.js

// context is a 2d canvas context
var oncall = require('./oncall.js')

var calls = oncall(context)
calls.on('fillRect', function(x, y, width, height) {
  console.log(context.fillStyle) // 'blue'
  console.log(x, y, width, height) // 0, 0, 200, 200
})

fakeContext.fillStyle = 'blue'
fakeContext.fillRect(0, 0, 200, 200)
fakeContext.fillStyle = 'green'