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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?
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Because == (and === ) test to see if two objects are the same object and not if they are identical objects.
The reason for [] == false even though [] is truthy is: the comparison [] == false compares the value of [] to false . And to get the value of [] , the JavaScript engine first calls []. toString() . That results in "" , and that is what's actually compared to false .
Double equals, == , performs an amount of type coercion on values before attempting to check for equality. So arr == arr returns true as you'd expect as what you are actually checking is if [] == [] and both sides of the equation are of the same type.
False == (False or True) In this expression python would first solve the comparison operator in bracket. => (False or True) is said to be True. Because the 'OR' operator search for the first truthy value, and if found it returns the value else 'False'. Now we have: =>False == True is False.
See the ECMAScript standard:
11.4.9 Logical NOT Operator ( ! )
The production UnaryExpression : ! UnaryExpression is evaluated as follows:
- Let expr be the result of evaluating UnaryExpression.
- Let oldValue be ToBoolean(GetValue(expr)).
- If oldValue is true, return false.
- 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.
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