1. instanceof 运算符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 instanceOf(left, right) {
      2. let proto = left.__proto__;
      3. let prototype = right.prototyppe;
      4. while(true) {
      5.   if(proto === null) return false;
      6. if(proto === prototype) return true;
      7. proto = proto.__proto__;
      8. }
      9. }
      10. var a = 10;
      11. console.log(instanceOf(10, Number)); // true;
      12. console.log(instanceOf(10, String));  // false;