javascript - 字符串强制转换为 bool 值

Question

Boolean("a")

在浏览器控制台中返回 true。
那么为什么

"a" == true

返回错误？

Answer 1

== 运算符如何在某些类型上发挥作用在 ECMAScript specifications 中定义。 。如下:

7.2.13 Abstract Equality Comparison

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

1. If Type(x) is the same as Type(y), then Return the result of performing Strict Equality Comparison x === y.
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.
4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ! ToNumber(y).
5. If Type(x) is String and Type(y) is Number, return the result of the comparison ! ToNumber(x) == y.
6. If Type(x) is Boolean, return the result of the comparison ! ToNumber(x) == y.
7. If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).
8. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
9. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
10. Return false.

现在我们可以将它们应用到上面的案例中。首先将 bool 值转换为数字，然后尝试将字符串转换为数字(被解析为 NaN):

"a" == true
// Case 7 (true --> 1)
// =>"a" == 1
// Case 5 ("a" --> NaN)
// => NaN == 1
=> false