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js instanceof （2）

2024-09-03 05:56:32   218人阅读

instanceof运算符可以用来判断某个构造函数的prototype属性是否存在另外一个要检测对象的原型链上。
实例一：普遍用法

A instanceof B :检测B.prototype是否存在于参数A的原型链上.

1function Ben() {
2 
3}
4var ben = new Ben();
5console.log(ben instanceof Ben);//true

 实例二：继承中判断实例是否属于它的父类

1function Ben_parent() {}
2function Ben_son() {}
3Ben_son.prototype = new Ben_parent();//原型继承
4var ben_son = new Ben_son();
5console.log(ben_son instanceof Ben_son);//true
6console.log(ben_son instanceof Ben_parent);//true

 实例三：复杂用法

1function Ben() {}
2console.log(Object instanceof Object);     //true
3console.log(Function instanceof Function); //true
4console.log(Function instanceof Object);   //true
5console.log(Ben instanceof Function);      //true
6 
7console.log(String instanceof String);   //false
8console.log(Boolean instanceof Boolean); //false
9console.log(Ben instanceof Ben);         //false

看到上述的结果，是否有点懵了，究其原因，还需探其原理，下面我们来看看规范中如何定义的。

01ECMASCRIPT 5.1 Standard文档中的定义：
0211.8.6 The instanceof operator
03 
04The production RelationalExpression : RelationalExpression instanceof ShiftExpression is evaluated as follows:
05 
061.Let lref be the result of evaluating RelationalExpression.
072.Let lval be GetValue(lref).
083.Let rref be the result of evaluating ShiftExpression.
094.Let rval be GetValue(rref).
105.If Type(rval) is not Object, throw a TypeError exception.
116.If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
127.Return the result of calling the [[HasInstance]] internal method of rval with argument lval.
13 
1415.3.5.3 [[HasInstance]] (V)
15 
16Assume F is a Function object.
17 
18When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:
19 
20//V不是对象，直接返回false
211.If V is not an object, return false.
22 
23//用 [[Get]] 方法取 F方法的prototype属性，结果赋值给O
242.Let O be the result of calling the [[Get]] internal method of F with property name "prototype".
25 
26//如果O不是一个对象，报告异常
273.If Type(O) is not Object, throw a TypeError exception.
28 
29//重复循环
304.Repeat
31    //V = V.[[Prototype]]
32    a.Let V be the value of the [[Prototype]] internal property of V.
33    b.If V is null, return false.
34    c.If O and V refer to the same object, return true.
35 
36NOTE Function objects created using Function.prototype.bind have a different implementation of [[HasInstance]] defined in 15.3.4.5.3.

对应上述规范做个函数模拟A instanceof B：

01function _instanceof(A, B) {
02    var O = B.prototype;// 取B的显示原型
03    A = A.__proto__;// 取A的隐式原型
04    while (true) { 
05        //Object.prototype.__proto__ === null
06        if (A === null) 
07            return false; 
08        if (O === A)// 这里重点：当 O 严格等于 A 时，返回 true 
09            return true; 
10        A = A.__proto__;
11    }
12}

使用此函数模拟解析过程：

01Object instanceof Object解析,执行_instanceof (Object, Object)
02O = Object.prototype;
03A = Object.__proto__ = Function.prototype
04A = Function.prototype.__proto__ = Object.prototype
05return true;
06 
07Function instanceof Function解析,执行_instanceof (Function, Function)
08O = Function.prototype;
09A = Function.__proto__ = Function.prototype;
10return true;
11 
12Function instanceof Object解析,执行_instanceof (Function, Object)
13O = Object.prototype;
14A = Function.__proto__ = Function.prototype;
15A = Function.prototype.__proto__ = Object.prototype;
16return true;
17 
18String instanceof String解析,执行_instanceof (String, String)
19O = String.prototype;
20A = String.__proto__ = Function.prototype;
21A = Function.prototype.__proto__ = Object.prototype;
22A = Object.prototype.__proto__ = null;
23return false;
24 
25Ben instanceof Ben解析,执行_instanceof (Ben, Ben)
26O = Ben.prototype;
27A = Ben.__proto__ = Ben.prototype;
28A = Ben.prototype.__proto__ = Object.prototype;
29A = Object.prototype.__proto__ = null;
30return false;

 参考链接：

http://www.zuojj.com/tdocs/es5.1/#sec-11.8.6  

http://blog.csdn.net/cuew1987/article/details/15498121

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/instanceof

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js instanceof （2）


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