SOURCE

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

const yoshi = {
  skulk : true
}
const hattori = {
  sneak : true
}
const kuma = {
  creep : true
}
assert('skulk' in yoshi, "123")


Object.setPrototypeOf(yoshi,hattori)
Object.setPrototypeOf(hattori,kuma)

assert('sneak' in yoshi,'12321212')
assert('creep' in yoshi,'21212')