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HomeWeb Front-endJS Tutorialjavascript instanceof 内部机制探析_javascript技巧

比如:

复制代码 代码如下:

// 代码 1
function Pig() {}
var pig = new Pig();
alert(pig instanceof Pig); // => true

function FlyPig() {}
FlyPig.prototype = new Pig();
var flyPig = new FlyPig();
alert(flyPig instanceof Pig); // => true

来看另一段代码:
复制代码 代码如下:

// 代码 2
function Pig() { Pig.prototype = {/* some code */} }
var pig = new Pig();
alert(pig instanceof Pig); // => false

为何上面的猪 pig 不再是猪 Pig 了呢?
当一个对象是某个类的实例时,意味着这个对象具有该类的方法和属性。在 JavaScript 中,一个猪类的特性体现在原型中:
复制代码 代码如下:

// 代码 3
function Pig() {}
Pig.prototype = {
"吃猪食": function() {},
"睡觉": function() {},
"长膘": function() {}
};
var pig = new Pig();
alert(pig instanceof Pig); //=> true

如果动态改变了猪的特性,让猪变成了牛:
复制代码 代码如下:

// 代码 4
Pig.prototype = {
"吃草": function() {},
"犁田": function() {}
};
var niu= new Pig();
alert(pig instanceof Pig); //=> false
alert(niu instanceof Pig); //=> true

当未改变 Pig 的 prototype 时,猪还是猪,因此代码 3 中 pig 是 Pig 的实例。当改变 prototype 后,猪已经不是猪,而是披着猪皮的牛了。因此代码 4 中 pig 不再是 Pig 的实例,niu 反而是 Pig 的实例。

进一步分析前,先回顾一下 new 的内部机制。代码 2 中的 new Pig() 实际上等价为:
复制代码 代码如下:

// var pig = new Pig() 的等价伪代码:
var pig = (function() {
var o = {};
o.__proto__ = Pig.prototype; // line 2
Pig.call(o);
Pig.prototype = {/* some code */}; // line 4
return o; // line 5
})();

可以看出,在 line 2 时,o.__proto__ 指向了 Pig.prototype 指向的值。但在 line 4 时,Pig.prototype 指向了新值。也就是说,在 line 5 返回时,pig.__proto__ !== Pig.prototype. 正是这个变化,导致了代码 2 中的 pig 不是 Pig.

已经可以大胆推论出:instanceof 判断 pig 是不是 Pig 的依据是:看隐藏的 pig.__proto__ 属性是否等于 Pig.prototype !

为了进一步确认,我们可以在 Firefox 下模拟 instanceof 的内部实现代码:
复制代码 代码如下:

/**
* Gecko 引擎下,模拟 instanceof
*/
function _instanceof(obj, cls) {
// instanceof 的左操作数必须是非null对象或函数对象
if((typeof obj !== "object" || obj === null)
&& typeof obj !== "function") {
return false;
}

// instanceof 的右操作数必须是函数对象
if(typeof cls !== "function") {
throw new Error("invalid instanceof operand (" + cls + ")");
}

// 向上回溯判断
var p = obj.__proto__, cp = cls.prototype;
while(p) {
if(p === cp) return true;
p = p.__proto__;
}
return false;
}

测试页面:simulate-intanceof.html

最后考考大家:
复制代码 代码如下:

function Bird() {}
var bird = new Bird();
var o = {};
bird.__proto__ = o;
Bird.prototype = o;
alert(bird instanceof Bird); // true or false?
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