为什么JavaScript中有2 == [2]？

问题描述：

我最近在JavaScript中发现 2 == [2] 。事实证明，这个怪癖有几个有趣的后果：

I recently discovered that 2 == [2] in JavaScript. As it turns out, this quirk has a couple of interesting consequences:

var a = [0, 1, 2, 3];
a[[2]] === a[2]; // this is true

同样，以下作品：

var a = { "abc" : 1 };
a[["abc"]] === a["abc"]; // this is also true

更奇怪的是，这也有效：

Even stranger still, this works as well:

[[[[[[[2]]]]]]] == 2; // this is true too! WTF?

这些行为似乎在所有浏览器中都是一致的。

These behaviors seem consistent across all browsers.

知道为什么这是一种语言功能吗？

Any idea why this is a language feature?

以下是这个功能的更为疯狂的后果：

Here are more insane consequences of this "feature":

[0] == false // true
if ([0]) { /* executes */ } // [0] is both true and false!

var a = [0];
a == a // true
a == !a // also true, WTF?

这些例子来自jimbojw http://jimbojw.com 成名以及 walkingeyerobot 。

These examples were found by jimbojw http://jimbojw.com fame as well as walkingeyerobot.

答

您可以在ECMA规范中查找比较算法（ECMA-262的相关章节，第3版针对您的问题：11.9.3， 9.1,8.6.2.6）。

You can look up the comparison algorithm in the ECMA-spec (relevant sections of ECMA-262, 3rd edition for your problem: 11.9.3, 9.1, 8.6.2.6).

如果将所涉及的抽象算法转换回JS，那么在评估 2 == [2]时会发生什么？ / code>基本上是这样的：

If you translate the involved abstract algorithms back to JS, what happens when evaluating 2 == [2] is basically this:

2 === Number([2].valueOf().toString())

其中 valueOf（）对于数组返回数组本身，单元素数组的字符串表示形式是单个元素的字符串表示。

where valueOf() for arrays returns the array itself and the string-representation of a one-element array is the string representation of the single element.

这也解释了第三个例子为 [[[[[[[2]]]]]]]。toString（）仍然只是字符串 2 。

This also explains the third example as [[[[[[[2]]]]]]].toString() is still just the string 2.

As你可以看到，有相当多的幕后魔术，这就是为什么我一般只使用严格的相等运算符 === 。

As you can see, there's quite a lot of behind-the-scene magic involved, which is why I generally only use the strict equality operator ===.

第一个和第二个例子更容易理解，因为属性名称总是字符串，所以

The first and second example are easier to follow as property names are always strings, so

a[[2]]

相当于

a[[2].toString()]

只是

a["2"]

请记住，在任何数组魔法发生之前，偶数数字键也会被视为属性名称（即字符串）。

Keep in mind that even numeric keys are treated as property names (ie strings) before any array-magic happens.

为什么JavaScript中有2 == [2]？

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