为什么 2 == [2] 在 JavaScript 中? [英] Why does 2 == [2] in JavaScript?

问题描述

我最近发现 JavaScript 中的 2 == [2].事实证明，这种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

类似地，以下工作:

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?

这些行为在所有浏览器中似乎都是一致的.

These behaviors seem consistent across all browsers.

知道为什么这是一个语言功能吗?

Any idea why this is a language feature?

这里是这个功能"的更疯狂的后果:

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?

推荐答案

您可以在 ECMA-spec 中查找比较算法(ECMA-262, 3rd edition 的相关部分针对您的问题: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).

如果将涉及的抽象算法翻译回 JS，则在评估 2 == [2] 时会发生什么基本上是这样的:

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

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

其中数组的 valueOf() 返回数组本身，单元素数组的字符串表示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，因为[[[[[[[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.

如您所见，其中涉及到很多幕后魔法，这就是为什么我通常只使用严格相等运算符 ===.

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()]

这只是

a["2"]

请记住，在任何数组魔术发生之前，即使是数字键也被视为属性名称(即字符串).

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

这篇关于为什么 2 == [2] 在 JavaScript 中?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！