为什么空数组类型转换为零?+[]

2022-01-12 00:00:00 type-conversion javascript

就在我以为我对 JavaScript 中的类型转换有所了解时，我偶然发现了这个:

just when I thought I understood something about type conversion in JavaScript, I stumbled with this:

+[]; // 0
Number([]); // 0

我的第一个想法是我应该得到 NaN，就像我尝试将空对象转换为数字一样:

My first thought was that I should get NaN, just like if I try to convert an empty object to number:

+{}; // NaN
Number({}); // NaN

我已经搜索了一段时间没有任何成功......

I have been searching about this for a while without any success...

有人能解释一下为什么它被转换为 0 而不是 NaN 吗?

Can somebody explain me why it gets converted to 0 and not to NaN?

这种行为标准吗?

谢谢.

推荐答案

简单来说有两个关键点:

In a brief, there are two key points:

  • 空数组的toString方法返回一个空字符串.
  • 使用 一元加运算符将空字符串 强制 归零 或作为函数调用的 Number 构造函数(例如 +"" === 0;).
  • The toString method of an empty array returns an empty string.
  • An empty string coerces to zero using the unary plus operator or the Number constructor called as a function (e.g. +"" === 0;).

例如，我们可以使用一个对象，它定义了一个toString方法，并返回一个空字符串得到一个等价的结果:

For example, we can use an object that defines a toString method, and returns an empty string to have an equivalent result:

var obj = { toString: function () { return "";} };
+obj; //  0
Number(obj); // 0

这种行为是完全标准的.

This behavior is completely standard.

现在是长答案:

一元加运算符和Number 构造函数作为函数调用 在内部使用 ToNumber 抽象操作.

Both, the unary plus operator and the Number constructor called as a function internally use the ToNumber abstract operation.

ToNumber 将使用另外两个内部操作，ToPrimitive 和 [[DefaultValue]].

ToNumber will use two more internal operations, ToPrimitive and [[DefaultValue]].

当 ToNumber 操作应用于对象时，例如您示例中的空数组，它调用 ToPrimitive 操作，以获取代表原始值并调用 ToNumber 再次使用该值 [1].

When the ToNumber operation is applied to an Object, such the empty array in your example, it calls the ToPrimitive operation, to get a representative primitive value and call ToNumber again using that value [1].

ToPrimitive 操作接收两个参数，一个 Value(这是您的数组对象)和一个提示类型，在本例中为Number"，因为我们要进行数字转换.

The ToPrimitive operation receive two arguments, a Value (which is your array object), and a hint type, which in this case is "Number" since we want to make numeric conversion.

ToPrimitive 调用 [[DefaultValue]] 内部方法，同样带有数字"提示类型.

ToPrimitive calls the [[DefaultValue]] internal method, also with a "Number" hint type.

现在，由于我们使用数字"的提示类型，[[DefaultValue]] 内部方法现在将尝试首先调用对象上的 valueOf 方法.

Now, since the hint type we are using "Number", the [[DefaultValue]] internal method now will try to invoke first the valueOf method on the object.

数组对象没有特定的 valueOf 方法，该方法是继承自 Object.prototype.valueOf，并且这个方法简单地返回一个对object本身的引用.

Array objects don't have a specific valueOf method, the method is the one inherited from Object.prototype.valueOf, and this method simply returns a reference to the object itself.

由于 valueOf 方法没有产生 原始值，现在调用 toString 方法，它产生一个空字符串(这是一个原始值)，然后 ToNumber 操作将尝试进行 String-Number 转换，最终以 0 [1].

Since the valueOf method didn't result in a primitive value, now the toString method is invoked, and it produces an empty string (which is a primitive value), then the ToNumber operation will try to do String-Number conversion and it finally end up with 0 [1].

但现在您可能想知道，为什么一个空字符串会强制为零?

But now you might wonder, why an empty string coerces to zero?

+""; // 0

ToNumber内部操作应用于 String 类型，即 StringNumericLiteral 产生式.

There is a complete grammar that is used when the ToNumber internal operation is applied to a String type, the StringNumericLiteral production.

NumericLiteral 之间存在一些差异，其中一个差异是:

It has some differences between a NumericLiteral, and one of those differences is that:

为空或仅包含空格的 StringNumericLiteral 将转换为 +0.

A StringNumericLiteral that is empty or contains only white space is converted to +0.

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