Logical NOT

The logical NOT operator is !. The logical NOT operator may be applied to any value in JavaScript. This operator always returns a Boolean value regardless of the data type.

The logical NOT operator first converts the operand to a Boolean value and then negates it.

The following table lists the data types and its corresponding result for logical not operator:

Operand	Result
an object	false
empty string("")	true
nonempty string	false
0	true
any number other than 0 (including Infinity)	false
null	true
NaN	true
undefined	true

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
    
        document.writeln(!false); //true 
        document.writeln(!"asdf"); //false 
        document.writeln(!0); //true 
        document.writeln(!NaN); //true 
        document.writeln(!""); //true 
        document.writeln(!12345); //false 

       
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

Boolean() vs !!

By using two NOT operators(!!), you can simulate the behavior of the Boolean() casting function.

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
        document.writeln(!!"asdf"); //true 
        document.writeln(!!0); //false 
        document.writeln(!!NaN); //false 
        document.writeln(!!""); //false 
        document.writeln(!!12345); //true 
    </script>
</head>
<body>
</body>
</html>

Click to view the demo