Question

Was just playing around with nodejs and chrome’s console when I tested this:

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

How come? Isn’t it wrong?

Answer 1

See the 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

An array is an Object.