javascript - 为什么 ![] 不是真的？

Question

当我测试这个时，我只是在玩弄 nodejs 和 chrome 的控制台:

[] == true // false
![] == true // false
!![] == true // true

怎么会？是不是错了？

Answer 1

参见 ECMAScript standard :

11.4.9 Logical NOT Operator ( ! )

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Let oldValue be ToBoolean(GetValue(expr)).
3. If oldValue is true, return false.
4. Return true.

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

• 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.
• The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
• Object → true

数组是一个对象。