为什么方法的`this`在调用对象方法的引用时会改变？ [英] Why does a method's `this` change when calling a reference to an object's method?

问题描述

function Person(gender) {
  this.gender = gender;
}

Person.prototype.sayGender = function()
{
  alert(this.gender);
};

var person1 = new Person('Male');
var genderTeller = person1.sayGender;

person1.sayGender(); // alerts 'Male'
genderTeller(); // alerts undefined

为什么genderTeller（）;警报undefined对我来说并不清楚。如果我看到它我相信它就像它上面的线一样。有些人可以解释详细信息

Why does genderTeller(); alerts undefined is not clear to me. if I see it I believe it's just same as line above it. Can some please explain the details

推荐答案

当你指定一个这样的变量时......

When you assign a variable like this...

var genderTeller = person1.sayGender;

...你丢失了上下文 > person1 object，函数的 this 指向全局对象（ window in a浏览器），而不是实例化的 person1 对象。

...you lose the context of the person1 object, and the function's this points to the global object (window in a browser), instead of the instantiated person1 object.

你得到 undefined 因为窗口上不存在 gender 属性，并且引用对象上的未定义属性返回< JavaScript中的code> undefined 。

You get undefined because the gender property does not exist on window, and referencing an undefined property on an object returns undefined in JavaScript.

您可以使用 bind（） ...

You can fix that in modern browsers with bind()...

var genderTeller = person1.sayGender.bind(person1);

...或jQuery有一个方法，也称为 proxy（） 。

...or jQuery has a method too called proxy().

var genderTeller = $.proxy(person1.sayGender, person1);

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