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 truthy. If a value can be converted to false, the value is so-called falsy.

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 constructor.

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

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 "truthiness" or "falsiness" of the value (see falsy).

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

The following operation involving booleans:

!!bCondition

is always equal to:

bCondition

• Boolean
• Truthy
• Falsy