Javascript - Operator Boolean NOT

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Introduction

The logical NOT operator is ! and can be applied to any value in ECMAScript.

This operator always returns a Boolean value, regardless of the data type it's used on.

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

InputNot Input
an object false is returned.
an empty string true is returned.
a nonempty string false is returned.
number 0 true is returned.
any number other than 0 (including Infinity) false is returned.
null true is returned.
NaN true is returned.
undefined true is returned.

The following example illustrates this behavior: 

console.log(!false);      //true 
console.log(!"blue");     //false 
console.log(!0);          //true 
console.log(!NaN);        //true 
console.log(!"");         //true 
console.log(!12345);      //false

The logical NOT operator can convert a value into its Boolean equivalent.

By using two NOT operators in a row, you can effectively simulate the behavior of the Boolean() casting function. 

console.log(!!"blue");     //true 
console.log(!!0);          //false 
console.log(!!NaN);        //false 
console.log(!!"");         //false 
console.log(!!12345);      //true