JavaScript Unary Operators The logical NOT operator (!)
Example
The logical NOT (!) operator performs logical negation on an expression.
Syntax:
!expression
Returns:
- a
Boolean.
Description
The logical NOT (!) operator performs logical negation on an expression.
Boolean values simply get inverted: !true === false and !false === true.
Non-boolean values get converted to boolean values first, then are negated.
This means that a double logical NOT (!!) can be used to cast any value to a boolean:
!!"FooBar" === true
!!1 === true
!!0 === false
These are all equal to !true:
!'true' === !new Boolean('true');
!'false' === !new Boolean('false');
!'FooBar' === !new Boolean('FooBar');
![] === !new Boolean([]);
!{} === !new Boolean({});
These are all equal to !false:
!0 === !new Boolean(0);
!'' === !new Boolean('');
!NaN === !new Boolean(NaN);
!null === !new Boolean(null);
!undefined === !new Boolean(undefined);
Examples:
!true // false
!-1 // false
!"-1" // false
!42 // false
!"42" // false
!"foo" // false
!"true" // false
!"false" // false
!{} // false
![] // false
!function(){} // false
!false // true
!null // true
!undefined // true
!NaN // true
!0 // true
!"" // true
