JavaScript:使用点符号和方括号符号时的行为不同 [英] JavaScript: Different behavior when using dot-notation vs. bracket-notation

问题描述

此JavaScript代码段

This JavaScript snippet

var x = window.foo;
window.x = null;
alert( window.bar === undefined );

提示"true".

但是，此代码段

var x = window.foo;
window[x] = null;
alert( window.bar === undefined );

提示"false".

这是怎么回事?

(我正在HTML页面中最新的Chrome浏览器中运行此代码，其中没有其他JavaScript代码.)

(I am running this code in the latest Chrome browser inside a HTML page with no other JavaScript code in it.)

更新

Update

@elusive在下面的评论中巧妙地总结了一下，我误认为 window.x 和 window [x] 是等效的.那是不对的. window.x 等同于 window ["x"] .

As @elusive cleverly summed up in his comment below, I mistakingly assumed that window.x and window[x] are equivalent. That is not correct. window.x is equivalent to window["x"].

推荐答案

您遇到的行为是因为Global对象的 undefined 属性在任何 ECMAScript 3 的实现.(最新的Chrome版本正在实现ES5，但此行为仍然存在).

The behavior that you are experiencing is because the undefined property of the Global object, is mutable on any ECMAScript 3 based implementation. (latest Chrome versions are implementing ES5, but this behavior is still present).

让我们检查第二个片段:

Let's examine the second snippet:

var x = window.foo;
window[x] = null;
alert( window.bar === undefined );

由于 foo 属性不存在，因此 x 变量将保留 undefined 值.

The x variable will hold the undefined value, since the foo property does not exist.

通过分配 window [x] = null ，您将覆盖 undefined 属性的值:

By assigning window[x] = null, you are overriding the value of the undefined property:

window[x] = null; // is equivalent to
window['undefined'] = null; // or
window.undefined = null; //

(在第一个代码段中，当您分配 window.x = null 时，您正在 window "x" 的属性>对象.)

(In your first snippet, when you assign window.x = null, you are creating a property named "x" on the window object.)

因此(在第二段代码中)， undefined 属性将保留 null ，而 window.bar 将产生 undefined :

Therefore (in your second snippet), the undefined property will hold null, and window.bar will produce undefined:

alert( window.bar === undefined ); // false
alert( undefined  === null ); // false

未在ECMAScript 3上将 undefined 属性未指定为 {ReadOnly} ((以及他的朋友 NaN ， Infinity ).

The undefined property was not specified as { ReadOnly } on ECMAScript 3, (along with his friends NaN, Infinity).

这在ECMAScript 5中已更改，这些属性被描述为不可写.

This has changed in ECMAScript 5, those properties are described as non-writables.

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