instanceof：用于检测构造函数的 prototype 属性是否出现在某个实例对象的原型链上

    1. function myInstanceof(target, origin) {
    2.   // 先判断target
    3.   if (
    4.     (typeof target !== 'function' && typeof target !== 'object') ||
    5.     target === null
    6.   )
    7.     return false
    9.   if (typeof origin !== 'function')
    10.     throw new TypeError('origin must be a function')
    12.   let proto = Object.getPrototypeOf(target) // proto === target.__proto__
    13.   while (proto) {
    14.     if (proto === origin.prototype) return true
    15.     // 按原型链查找
    16.     proto = Object.getPrototypeOf(proto)
    17.   }
    19.   return false
    20. }
    22. function foo() {}
    24. console.log(myInstanceof(foo, Object))
    25. console.log(foo instanceof Object)