为什么是“这个”在使用严格的匿名函数undefined？ [英] Why is "this" in an anonymous function undefined when using strict?

问题描述

为什么在严格模式下使用javascript时，匿名函数中的 this 是未定义的？我理解为什么这可能有意义，但我找不到任何具体的答案。

Why is this in an anonymous function undefined when using javascript in strict mode? I understand why this could make sense, but I couldn't find any concrete answer.

示例：

(function () {
    "use strict";

    this.foo = "bar"; // *this* is undefined, why?
}());

小提琴测试： http://jsfiddle.net/Pyr5g/1/
查看记录器（firebug）。

Test in a fiddle: http://jsfiddle.net/Pyr5g/1/ Check out the logger (firebug).

推荐答案

这是因为，直到ECMAscript 262第5版，如果人们使用构造函数模式，就会出现很大的混乱，忘了使用新关键字。如果在ES3中调用构造函数时忘记使用 new ，此引用了全局对象（窗口在浏览器中）你会用变量破坏全局对象。

It's because, until ECMAscript 262 edition 5, there was a big confusion if people who where using the constructor pattern, forgot to use the new keyword. If you forgot to use new when calling a constructor function in ES3, this referenced the global object (window in a browser) and you would clobber the global object with variables.

这是一种可怕的行为，所以ECMA的人决定，只需将此设置为 undefined 。

That was terrible behavior and so people at ECMA decided, just to set this to undefined.

示例：

function myConstructor() {
    this.a = 'foo';
    this.b = 'bar';
}

myInstance     = new myConstructor(); // all cool, all fine. a and b were created in a new local object
myBadInstance  = myConstructor(); // oh my gosh, we just created a, and b on the window object

最后一行是在ES5严重中抛出错误

The last line would throw an error in ES5 strict

"TypeError: this is undefined"

（这是一个更好的行为）

(which is a much better behavior)

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