The logical NOT (!) (logical complement, negation) operator takes truth to
falsity and vice versa. It is typically used with boolean (logical)
values. When used with non-Boolean values, it returns false if its single
operand can be converted to true; otherwise, returns true.
Syntax
!x
Description
Returns false if its single operand can be converted to true;
otherwise, returns true.
If a value can be converted to true, the value is so-called
. If a value can be converted to false, the value is
so-called
.
Examples of expressions that can be converted to false are:
null;NaN;0;- empty string (
""or''or``); undefined.
Even though the ! operator can be used with operands that are not Boolean values, it can still be considered a boolean operator since its return value can always be converted to a boolean primitive. To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator (!!) or the Boolean constructor.
Examples
Using NOT
The following code shows examples of the ! (logical NOT) operator.
!true; // !t returns false
!false; // !f returns true
!""; // !f returns true
!"Cat"; // !t returns false
Double NOT (!!)
It is possible to use a couple of NOT operators in series to explicitly force the conversion of any value to the corresponding boolean primitive. The conversion is based on the "truthyness" or "falsyness" of the value (see
and ).
The same conversion can be done through the Boolean() function.
!!true; // !!truthy returns true
!!{}; // !!truthy returns true: any object is truthy...
!!new Boolean(false); // ...even Boolean objects with a false .valueOf()!
!!false; // !!falsy returns false
!!""; // !!falsy returns false
!!Boolean(false); // !!falsy returns false
Converting between NOTs
The following operation involving booleans:
!!bCondition
is always equal to:
bCondition