JavaScript:为什么是 []==![]？

Question

在 JavaScript 中，我注意到 []==![] 结果是 true。 demo

我不明白这个结果。你能解释一下为什么吗？

Answer 1

[] 是一个数组，但是 ![] 是一个 bool 值。每当您尝试使用 == 比较两个不同类型的对象时，这两个对象都应转换为可比较的对象(使用 ToNumber，请参见 11.9.3 中的步骤 7)。这就是为什么 [] == ![] 产生 true，第一个空数组被评估为 false。

11.9.3 The Abstract Equality Comparison Algorithm

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

1. [...]
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 or Number 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 or Number, return the result of the comparison ToPrimitive(x) == y.
10. Return false.

但是，如果您使用严格类型比较运算符 ===，则结果为 false，因为两种类型不同:

11.9.6 The Strict Equality Comparison Algorithm

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 different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is Number, then
   1. If x is NaN, return false.
   2. If y is NaN, return false.
   3. If x is the same Number value as y, return true.
   4. If x is +0 and y is -0, return true.
   5. If x is -0 and y is +0, return true.
   6. Return false.
5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
7. Return true if x and y refer to the same object. Otherwise, return false.