JavaScript:instanceof 运算符

Question

第一个代码:

function MyConstructor() {}
var myobject = new MyConstructor();
MyConstructor.prototype = {};

[ myobject instanceof MyConstructor,   // false - why?
myobject.constructor == MyConstructor, // true
myobject instanceof Object ]           // true

即使 MyConstructor.prototype 被替换 myobject 仍然继承 Myconstructor.prototype 的属性。那么为什么 myobject instanceOf Myconstuctor 是假的呢？

function MyConstructor() {}
MyConstructor.prototype = {};
var myobject = new MyConstructor();
myobject instanceof MyConstructor  // true (it is because myobject still inherits from
                                   // Myconstructor.prototype although it has been replaced)

第二个:

 function MyConstructor() {}
 MyConstructor.prototype = {};
 var myobject = new MyConstructor();

 myobject.constructor == MyConstructor;  // false (accepted )

所以如果 myobject.constructor 是 Object 为什么第一个:示例不指向它，myobject.constructor 怎么可能仍然指向 MyConstructor 因为 Myconstructor.prototype 在第一个示例中发生了变化。

你能澄清一下吗？

Answer 1

even though MyConstructor.prototype is replaced myobject still inherits the properties from Myconstructor.prototype.

没有。它继承自被替换的旧对象。由于该对象是 !== MyConstructor.prototype，instanceof operator会产生错误。在您的第二个示例中，myobject 继承自新原型(prototype)(当前的 MyConstructor.prototype)，这就是 instanceof 告诉您的内容。

So if myobject.constructor

constructor 属性与 instanceof 完全无关。

function Constructor() {}
var oldProto = Constructor.prototype;
var oldInstance = new Constructor();

Constructor.prototype = {constructor:"something else"};
var newProto = Constructor.prototype;
var newInstance = new Constructor();

// all these are true:
Object.getPrototypeOf(oldInstance) === oldProto;
Object.getPrototypeOf(newInstance) == newProto;
oldProto !== newProto;
oldProto.constructor === Constructor; // was set implicitly on creating the function
oldInstance.constructor === oldProto.constructor; // inherited
newProto.constructor === "something else"; // if not explicitly set, comes from Object.prototype
newInstance.constructor === newProto.constructor; // inherited

Constructor.prototype === newProto;
newInstance instanceof Constructor; // because the above

Constructor.prototype !== oldProto;
! (oldInstance instanceof Constructor) // because the above