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Logical NOT (!)

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.

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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 {{Glossary("truthy")}}. If a value can be converted to false, the value is so-called {{Glossary("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 primitive. To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator (!!) or the {{jsxref("Boolean/Boolean", "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 {{Glossary("truthy")}} and {{Glossary("falsy")}}).

The same conversion can be done through the {{jsxref("Boolean/Boolean", "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

Specifications​

Browser compatibility​

See also​

  • {{jsxref("Boolean")}}
  • {{Glossary("Truthy")}}
  • {{Glossary("Falsy")}}