Using the (!!!) operator in JavaScript?

Question

!! operator is obviously useful as it forces to check something that's not a boolean as a Boolean - that's for "True" conditions.

But what about false? Do you need to use !!!?

! converts a value to the inverse boolean (if it was truthy it becomes false,falsy it becomes true) — megawac, Nov 11 '13 at 15:43
@Simon: I have never seen `====`. Assuming you mean strict comparison, how is a Boolean operation the opposite of comparison? — Felix Kling, Nov 11 '13 at 16:02
@GuilhermeSehn You should clarify that !value would suffice for a boolean conversion. — geoyws, May 19 '14 at 15:50
@Emissary People do come by !!! in other people's code and they need reassurance for their sanity. For example, Node.js (downloaded 5 days ago) uses !!!value in their assert.js. [Github shows that they've changed it in July 2013 though.](https://github.com/joyent/node/commit/dc9acd4faeba1aade414bdd8da28f30b16773575) Why does my version of Node not reflect this? — geoyws, May 19 '14 at 15:50
There is no `(!!!)` operator in JavaScript. `(!!!) true` → `Exception: expected expression, got ')'` — Nicolas Barbulesco, May 12 '15 at 18:19

score 4 · Answer 1 · answered Nov 11 '13 at 15:43

4

One ! would be enough :)

!1 // false
!0 // true

answered Nov 11 '13 at 15:43

andbas

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No, not if something comes back as undefined. !! forces undefined to be converted to true. – antonpug Nov 11 '13 at 15:47
1

@antonpug: You sure about that? http://jsfiddle.net/kkf7Y/ A single `!` seems to work just fine, at least in my browser. – David Nov 11 '13 at 15:49
http://stackoverflow.com/questions/4686583/can-someone-explain-this-double-negative-trick – antonpug Nov 11 '13 at 15:50
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@antonpug ok, the link you posted only show that I'm right. !! will never return true on undefined. Single ! convert any value to Boolean and apply the NOT operation to it, second ! will just apply one more NOT. – andbas Nov 11 '13 at 21:55

score 3 · Answer 2 · answered Nov 11 '13 at 16:03

3

!! 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.

answered Nov 11 '13 at 16:03

PleaseStand

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This answer is perfect and very clear. – Nicolas Barbulesco May 12 '15 at 18:25

Felix Kling · Answer 3 · 2013-11-11T16:02:35.687

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.

Bakudan · Answer 4 · 2013-12-01T12:48:46.910

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:

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

score -1 · Answer 5 · edited May 23 '17 at 10:24

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.

Using the (!!!) operator in JavaScript?

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