javascript: Meaning of two exclamation marks(!!)

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I was confused a little when I found two exclamation marks in javascript code.
I make a note so that not forgot.

case: one exclamation mark

var a = true;
console.log(!a); //false
// "booblean" can also converted to a number using "Number".
console.log(Number(!a)); //0

If you want a number in return value rather than booblean(true/false), then use Number(1/0).

case: two exclamation mark

"true" into "not not", then "true" is returned.

var b = true;
console.log(!!b); //true

In the following code, if you use the exclamation mark, you will see that it is converted to booblean(true/false).

var c = undefined; // same if var c =0,null 
console.log(!!c); //false
//Object is true
var d = Object;
console.log(!!d); //true

Reference:What is the !! (not not) operator in JavaScript?

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