一直想知道js的 instanceof 是根据什么来判断两个对象的继承关系？ 会不会跟prototype有关，然后就试试了
如果觉得下面的代码比较长，请先看注释
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>intanceof.html</title>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<script type="text/javascript">
/*
A instanceof B
如果A.__proto__ === B.prototype 则返回true
*/
var proto = {
say:function(){
alert("my name is "+this.name);
}
}
var Parent = function(name){
this.name = name||"yan";
}
Parent.prototype = proto;
var Child = function(age){
this.age = age;
}
Child.prototype = proto;
var child = new Child(11);
var parent = new Parent();
//测试
console.group("instanceof是根据prototype来判断两个对象的继承关系");
console.log("parent instanceof Parent :"+(parent instanceof Parent));//true;
console.log("child instanceof Parent :"+(child instanceof Parent));//true;
console.log("parent instanceof Child :"+(parent instanceof Child));//true;
console.log("Child instanceof Parent :"+(Child instanceof Parent));//false;
console.groupEnd();
/* A instanceof B 沿着A的原型链查找 如果有一个原型和B.prototype相等 则返回true 如：A.__proto__.__proto__ === B.prototype 则返回true */ var Fn = function(){} Fn.prototype = new Parent("a"); Child.prototype = new Fn(); var fn = new Fn(); child = new Child();
console.group("沿着原型链查找 如A instanceof B ，沿着A原型链查找直到找到跟B.prototype相等的原型");
console.log("child instanceof Parent :"+(child instanceof Parent));//true;
console.log("child instanceof Fn :"+(child instanceof Fn));//true;
console.log("fn instanceof Parent :"+(fn instanceof Parent));//true;
console.log("fn instanceof Child :"+(fn instanceof Child));//false;
console.groupEnd();
/* 进一步确定 instanceof 是检查原型的引用 而不是深入原型去检测他们的内容是否一样 */ var proto2 = { say:function(){ alert("my name is "+this.name); } } Child = function(){} Child.prototype = proto2; child = new Child();
console.group(" instanceof 是检查原型的引用 而不是深入比较原型的内容");
console.log("child instanceof Parent :"+(child instanceof Parent));//false;
console.groupEnd();
</script> </body> </html>
firefox执行结果如图
总结：
js的instanceof是根据prototype来判断两个对象是否存在继承关系，
A instanceof B
js会沿着A的原型链查找 直到找到A.__proto__ === B.prototype 返回true
期待你们的意见，谢谢。
注：__proto__属性指向prototype对象，对开发者不可见，例如var a = new A(), a.__proto__ = A.prototype;
