Using the (!!!) operator in JavaScript? [duplicate]

Answer 1

that's for "True" conditions.

No, it's for any value. !! will convert any value in it's equivalent Boolean value.

Why? Because the not operator simply returns the opposite Boolean value. So

0    // is falsy
!0   // -> true
!!0  // -> false

'foo'    // is truthy
!'foo'   // -> false
!!'foo'  // -> true

Any additional application will just toggle the value again. Thus !!! is equivalent to ! and !!!! is equivalent to !! and so on.

Answer 2

One ! would be enough :)

!1 // false
!0 // true

Answer 3

This answers your question perfectly https://stackoverflow.com/a/264037/1561922

!!!x is probably inversing a boolean conversion !!x:

var myBool = Boolean("false");  // == true

var myBool = !!"false";  // == true

"Any string which isn't empty will evaluate to true"

So !!!"false"; // == false

This question is NOT a joke. Node.js (downloaded 5 days ago) uses this in Assert.js for example:

function ok(value, message) {
  if (!!!value) fail(value, true, message, '==', assert.ok);
}
assert.ok = ok;

EDIT: I think they did it for code readability reasons out of habit, because !value already suffices.

EDIT: Node changed it. I have no idea why my version of Node.js that was downloaded 5 days ago is still with the !!!value instead of the !value in GitHub.

EDIT: This is the reason why.

Answer 4

var boolCheck = 0;
   boolCheck //0
  !boolCheck //true
 !!boolCheck //false
!!!boolCheck //true - again

Same thing here:

var boolCheck = 1;
   boolCheck //1
  !boolCheck //false
 !!boolCheck //true
!!!boolCheck //false - again

There is no reason to use this ( !!! ), it is a tautology. !! is useful when:

1. we need to have a Boolean value, when there are some results ( !!5 => true );
2. we need to shorten false with !1 or true - !0.
   1. or to skip the wrapper.

Answer 5

!! is not an operator in itself; it is merely a way of using JavaScript's logical NOT (!) operator.

To convert x to a boolean, !!x works because when you negate a boolean twice, you get the original boolean, and !x converts x to a boolean before negating it.

Likewise, !!!x converts x to a boolean and negates it three times, which is equivalent to negating it only once. So you can use !x instead of !!!x.