instanceof机制和实现

1. instanceof机制

1. 获取对象的__proto__和判断类型的prototype
2. 遍历对象上的所有原型直到==判断类型的prototype返回true
3. 若遍历到尽头也就是null了还是没结果返回false

function instanceof(left, right) {
  for (left = left.__proto__; left; left = left.__proto__) {
    if (left === right.prototype) return true
  }
  return false
}

2. [Symblol.hasInstance]方法重写

更精确的来说a instanceof B实际会执行B[Symbol.hasInstance](a)

// instanceof机制
class A {
  static [Symbol.hasInstance](instance){
    for(instance = instance.__proto; instance; instance = instance.__proto__){
      if(instance === this) return true
    }
    return false
  }
}
new A() instanceof A // true

// 重写
class Person{ 
  static [Symbol.hasInstance](instance){ 
    return instance instanceof Array 
  } 
} 

[1,2,3] instanceof Person // true
new Person() instanceof Person // false