Javascript：为什么“这个”私有函数里面指的是全局范围？ [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'指的是全局范围？为什么它不是函数'A'的范围？
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有一个不同的概念，特殊名称这个指的是比其他大多数编程语言都要多的
。正好有五种不同的
方式，其中此的值可以用该语言绑定。
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;
在全球范围内使用此时，它将简单地引用全局对象。
When using this in global scope, it will simply refer to the global object.
foo();
此处此将再次引用全球对象。
ES5注意：在严格模式下，全局案例<强>不再存在。
在这种情况下，此的值将改为undefined。
ES5 Note: In strict mode, the global case no longer exists.
thiswill instead have the value ofundefinedin that case.
调用方法
Calling a Method
test.foo();
在此示例中，此将引用 test 。
new foo();
一个函数调用，前面是 new keyword作为
a构造函数。在函数内部，此将
引用到新创建的 对象。
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
使用<$ c时$ c> call 或 apply Function.prototype 的方法，$ b的值$ b 调用函数内的将显式设置转换为相应函数调用的第一个参数
。
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，在上面的示例中，方法案例 不适用，此 $ b $ c里面的
foo 将设置为 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：
此无法用于引用对象
文字内的对象。所以var obj = {me：this}将不导致me引用
obj，因为此仅受列出的五个案例之一的约束。
Note:
thiscannot be used to refer to the object inside of anObjectliteral. Sovar obj = {me: this}will not result inmereferring toobj, sincethisonly gets bound by one of the five listed cases.
普通陷阱
虽然大多数情况都有意义，但第一个被认为是另一个
错误设计了该语言，因为从不有任何实际用途。
Foo.method = function() {
function test() {
// this is set to the global object
}
test();
}
一个常见的误解是这个在里面指的是 Foo ;而在
的事实上，它不。
A common misconception is that this inside of test refers to Foo; while in
fact, it does not.
为了获得 Foo 从 test 中，有必要在方法中创建
局部变量指 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();
}
只是一个普通的变量名，但它通常用于引用
外这个。与闭包相结合，它还可以
用于传递此值。
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.
不在JavaScript中工作的另一件事是函数别名，即
分配方法变量。
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，现在测试就像一个简单的函数调用;因此，
里面的这个将不再引用 someObject 。
Due to the first case, test now acts like a plain function call; therefore,
this inside it will no longer refer to someObject.
虽然这个的后期绑定起初可能看起来不是一个坏主意，但实际上，这是使原型继承有效的原因。
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();
当在一个实例上调用方法时 Bar ，此现在将引用该
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： Shamelessy从我自己的资源中偷走 http://bonsaiden.github.com/JavaScript-Garden/#function.this
Disclaimer: Shamelessy stolen from my own resources at http://bonsaiden.github.com/JavaScript-Garden/#function.this
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