SOURCE

function Ninja() {
  this.swung = false
  this.swingSword = function() {
    return !this.swung
  }
}
function assert(value,text) {
  if(value === true) {
    console.log(text)
  } else {
    console.log('error')
  }
}
Ninja.prototype.swingSword = function() {
  return this.swung
}

const ninja = new Ninja()
ninja.swingSword = function() {
  return '123'
}
assert(ninja.swingSword() === '123','called the instance method,not the prototype method')