SOURCE

function Ninja() {
  let _skillLevel = 0
  
  Object.defineProperty(this, "skillLevel", {
    get: () => _skillLevel,
    set: value => {
    if(!Number.isInteger(value)) {
    throw new TypeError("Skill level should be a number")
  		}
    _skillLevel = value
  	}
  })
}

const ninja = new Ninja()

console.log(ninja.skillLevel)

ninja.skillLevel = 10

console.log(ninja.skillLevel)

try {
  ninja.skillLevel = "Great"
  console.log("should not be here")
} catch(e) {
  console.log("Setting a non_integer value throw an exception")
}