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js instanceof （2）

时间：2017-02-06 12:46:22 阅读：184 评论：0 收藏：0 [点我收藏+]

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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.

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function Ben() {

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}

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var ben = new Ben();

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console.log(ben instanceof Ben);//true

实例二：继承中判断实例是否属于它的父类

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function Ben_parent() {}

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function Ben_son() {}

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Ben_son.prototype = new Ben_parent();//原型继承

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var ben_son = new Ben_son();

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console.log(ben_son instanceof Ben_son);//true

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console.log(ben_son instanceof Ben_parent);//true

实例三：复杂用法

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function Ben() {}

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console.log(Object instanceof Object);     //true

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console.log(Function instanceof Function); //true

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console.log(Function instanceof Object);   //true

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console.log(Ben instanceof Function);      //true

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console.log(String instanceof String);   //false

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console.log(Boolean instanceof Boolean); //false

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console.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。

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ECMASCRIPT 5.1 Standard文档中的定义：

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11.8.6 The instanceof operator

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The production RelationalExpression : RelationalExpression instanceof ShiftExpression is evaluated as follows:

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1.Let lref be the result of evaluating RelationalExpression.

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2.Let lval be GetValue(lref).

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3.Let rref be the result of evaluating ShiftExpression.

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4.Let rval be GetValue(rref).

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5.If Type(rval) is not Object, throw a TypeError exception.

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6.If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.

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7.Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

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15.3.5.3 [[HasInstance]] (V)

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Assume F is a Function object.

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When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:

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//V不是对象，直接返回false

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1.If V is not an object, return false.

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//用 [[Get]] 方法取 F方法的prototype属性，结果赋值给O

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2.Let O be the result of calling the [[Get]] internal method of F with property name "prototype".

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//如果O不是一个对象，报告异常

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3.If Type(O) is not Object, throw a TypeError exception.

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//重复循环

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4.Repeat

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    //V = V.[[Prototype]]

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    a.Let V be the value of the [[Prototype]] internal property of V.

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    b.If V is null, return false.

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    c.If O and V refer to the same object, return true.

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NOTE 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：

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function _instanceof(A, B) {

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    var O = B.prototype;// 取B的显示原型

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    A = A.__proto__;// 取A的隐式原型

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    while (true) { 

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        //Object.prototype.__proto__ === null

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        if (A === null) 

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            return false; 

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        if (O === A)// 这里重点：当 O 严格等于 A 时，返回 true 

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            return true; 

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        A = A.__proto__;

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    }

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}

使用此函数模拟解析过程：

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Object instanceof Object解析,执行_instanceof (Object, Object)

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O = Object.prototype;

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A = Object.__proto__ = Function.prototype

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A = Function.prototype.__proto__ = Object.prototype

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return true;

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Function instanceof Function解析,执行_instanceof (Function, Function)

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O = Function.prototype;

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A = Function.__proto__ = Function.prototype;

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return true;

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Function instanceof Object解析,执行_instanceof (Function, Object)

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O = Object.prototype;

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A = Function.__proto__ = Function.prototype;

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A = Function.prototype.__proto__ = Object.prototype;

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return true;

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String instanceof String解析,执行_instanceof (String, String)

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O = String.prototype;

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A = String.__proto__ = Function.prototype;

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A = Function.prototype.__proto__ = Object.prototype;

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A = Object.prototype.__proto__ = null;

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return false;

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Ben instanceof Ben解析,执行_instanceof (Ben, Ben)

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O = Ben.prototype;

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A = Ben.__proto__ = Ben.prototype;

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A = Ben.prototype.__proto__ = Object.prototype;

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A = Object.prototype.__proto__ = null;

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return false;