SOURCE

function assert(value, text) {
  if(value === true) {
    console.log(text)
  } else {
    console.log('error')
  }
}

function Ninja() {
  var feints = 0
  this.getFeint = function() {
    return feints
  }
  this.feint = () => {
    feints++
  }
}

var ninja1 = new Ninja()
ninja1.feint()

var imposter = {}

imposter.getFeint = ninja1.getFeint
console.log(imposter.getFeint() )
assert(imposter.getFeint() === 1, "123")