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。
A instanceof B 检查A.__proto__是否在B.prototype function myInstanceof(A, B) { A = A.__proto__; B = B.prototype; while (A) { if (A === B) { return true; } A = A.__proto__; } return false; } function C() { } function D() { } var o = new C(); console.log(myInstanceof(o, C));//true console.log(myInstanceof(o, D));//false console.log(myInstanceof(o, Object));//true console.log(myInstanceof(C.prototype, Object));//true