JS类型强制如何工作？ [英] How does JS type coercion work?

问题描述

我正在学习 == 与 === 并遇到此answer 对理解这个概念非常有帮助。但是我想知道其中一个例子：

I'm learning about == vs. === and came across this answer which was very helpful in understanding the concept. However I wondered about one of the examples:

'0' == false     // true

这可能有意义，因为 == 不会检查类型。但后来我在控制台中尝试了一些可能的强制措施，发现了以下内容：

This might make sense, since == doesn't check for type. But then I tried some possible coercions in the console and found the following:

Boolean('0')     // true
String(false)    // "false"

我原以为 '0'== false 具有与'0'=== String（false）相同的真值，但似乎没有是这样的。

I would have thought '0' == false has the same truth value as '0' === String(false), but that doesn't seem to be the case.

那么强制如何实际起作用？是否有一种我缺少的基本类型？

So how does the coercion actually work? Is there a more basic type I'm missing?

推荐答案

0是包含字符 0 的字符串，不数字值 0 。评估为 false 的唯一字符串类型值为。

"0" is a string containing the character 0, it is not the numeric value 0. The only string-type value which evaluates to false is "".

0 truthy 。

ECMAScript 262规范的第9.2节定义了不同类型如何转换为布尔值：

Section 9.2 of the ECMAScript 262 specification defines how different types are converted to Boolean:

Argument Type   Result
Undefined       false
Null            false
Boolean         The result equals the input argument (no conversion).
Number          The result is false if the argument is +0, −0, or NaN; otherwise the
                result is true.
String          The result is false if the argument is the empty String (its length is
                zero); otherwise the result is true.
Object          true

然而，只有在使用<$ c $进行比较时才会严格遵循c> === 。

当使用布尔值（'0'）时你'将值'0'转换为布尔值（与使用 !!'0'相同）。当松散地比较'0'与 false 时，布尔值将转换为数字（如定义的这里）。 false ，当转换为数字时，变为 0 。这意味着最终计算是'0'== 0 ，相当于 true 。

When using Boolean('0') you're converting the value '0' to Boolean (which is the same as using !!'0'). When loosely comparing '0' with false, the Boolean value is converted to a number (as defined here). false, when converted to a number, becomes 0. This means the final calculation is '0' == 0 which equates to true.

总结上述ECMAScript规范链接部分的相关部分：

To summarise the relevant part of the linked section of the ECMAScript specification above:

1. 让 x = '0'和 y = false 。


2. 检查 y 的类型是否为布尔值。


3. 如果为true，则将 y 转换为数字。


4. 将 x 与 y 的等效数字进行比较。

1. Let x = '0' and y = false.
2. Check if the type of y is Boolean.
3. If true, convert y to a number.
4. Compare x to the numeric equivalent of y.

在我们的例子中，JavaScript的实现方式是：

In our case, a JavaScript implementation of this would be:

var x = '0',                      // x = "0"
    y = false;                    // y = false

if (typeof y === "boolean") {
    y = +y;                       // y = 0
}

console.log( x == y );            // "0" == 0

-> true

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