Javascript:为什么“这个"私有函数里面是指全局作用域吗? [英] Javascript: why "this" inside the private function refers to the global scope?

问题描述

考虑以下代码:

function A() {}    

A.prototype.go = function() {
    console.log(this); //A { go=function()}

    var f = function() {
         console.log(this);  //Window              
    };

    f();
}

var a = new A();
a.go();

为什么函数 'f' 中的 'this' 指的是全局作用域?为什么不是函数'A'的作用域?

Why does 'this' inside function 'f' refers to the global scope? Why it is not the scope of function 'A' ?

推荐答案

JavaScript 对于特殊名称 this 所指的内容有不同的概念与大多数其他编程语言相比.正好有五个不同的this 的值可以在语言中绑定的方式.

JavaScript has a different concept of what the special name this refers to than most other programming languages do. There are exactly five different ways in which the value of this can be bound in the language.

this;

在全局范围内使用 this 时，它只会引用 global 对象.

When using this in global scope, it will simply refer to the global object.

foo();

这里，this 将再次引用全局对象.

Here, this will again refer to the global object.

ES5 注意:在严格模式下，全局大小写不再存在.在这种情况下，this 将改为具有 undefined 的值.

ES5 Note: In strict mode, the global case no longer exists. this will instead have the value of undefined in that case.

调用方法

test.foo(); 

在这个例子中，this 将引用 test.

In this example, this will refer to test.

new foo(); 

以 new 关键字开头的函数调用充当一个构造函数.在函数内部，this 将引用到一个新创建的 Object.

A function call that is preceded by the new keyword acts as a constructor. Inside the function, this will refer to a newly created Object.

function foo(a, b, c) {}

var bar = {};
foo.apply(bar, [1, 2, 3]); // array will expand to the below
foo.call(bar, 1, 2, 3); // results in a = 1, b = 2, c = 3

当使用Function.prototype的call或apply方法时，被调用函数内的 this 被显式设置到第一个参数对应的函数调用.

When using the call or apply methods of Function.prototype, the value of this inside the called function gets explicitly set to the first argument of the corresponding function call.

因此，在上面的例子中，方法案例不适用，而thisfoo 的内部将被设置为 bar.

As a result, in the above example the method case does not apply, and this inside of foo will be set to bar.

注意: this 不能用于引用Object内部的对象文字.所以 var obj = {me: this} 将 not 导致 me 引用obj，因为 this 只受到列出的五种情况之一的约束.

Note: this cannot be used to refer to the object inside of an Object literal. So var obj = {me: this} will not result in me referring to obj, since this only gets bound by one of the five listed cases.

常见的陷阱

虽然大多数情况都有意义，但第一种情况被视为另一种情况语言的错误设计，因为它从未有任何实际用途.

Foo.method = function() {
    function test() {
        // this is set to the global object
    }
    test();
}

一个常见的误解是test中的this指的是Foo；而在事实上，它没有.

A common misconception is that this inside of test refers to Foo; while in fact, it does not.

为了从 test 中访问 Foo，有必要创建一个method 内部的局部变量，引用 Foo.

In order to gain access to Foo from within test, it is necessary to create a local variable inside of method which refers to Foo.

Foo.method = function() {
    var that = this;
    function test() {
        // Use that instead of this here
    }
    test();
}

that 只是一个普通的变量名，但它通常用于引用一个外部 this.结合闭包，它还可以用于传递 this 值.

that is just a normal variable name, but it is commonly used for the reference to an outer this. In combination with closures, it can also be used to pass this values around.

在 JavaScript 中不的另一件事是函数别名，即为变量赋值方法.

Another thing that does not work in JavaScript is function aliasing, which is assigning a method to a variable.

var test = someObject.methodTest;
test();

由于第一种情况，test 现在就像一个普通的函数调用；所以，里面的this将不再引用someObject.

Due to the first case, test now acts like a plain function call; therefore, this inside it will no longer refer to someObject.

虽然 this 的后期绑定乍一看似乎是个坏主意，但在事实上，这就是原型继承起作用的原因.

While the late binding of this might seem like a bad idea at first, in fact, it is what makes prototypal inheritance work.

function Foo() {}
Foo.prototype.method = function() {};

function Bar() {}
Bar.prototype = Foo.prototype;

new Bar().method();

当在 Bar 的实例上调用 method 时，this 现在将引用那个非常实例.

When method gets called on a instance of Bar, this will now refer to that very instance.

免责声明:无耻从我在 http://bonsaiden 上的资源中窃取.github.com/JavaScript-Garden/#function.this

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