用法

function Person(name, age) {
  this.name = name
  this.age = age
  this.say = function () {
    console.log('my name is' + this.name)
  }
}

a = new Person('张三', 18)

a instanceof Person // true
a instanceof Function // false
a instanceof Object // true

实现

寻找left的对象中的原型链中是否存在right函数的原型对象（right.prototype）

function myInstanceof(left, right) {
  prototype = right.prototype
  while(true) {
    if (left.__proto__ === prototype) {
      return true
    } else if (left.__proto__ !== null) {
      left = left.__proto__
    } else {
      return false
    }
  }
}