Meaning of !! javascript

Question

I found some code about authentication with angular and i can't understand this trick :

authService.isAuthenticated = function () {
    return !!Session.userId;
};

What does !! mean 'different of userId' ?

whenever true = !!true = !!!!true =>etc, it don't understand this.

Somebody can help me?

(https://medium.com/opinionated-angularjs/techniques-for-authentication-in-angularjs-applications-7bbf0346acec for the source , part 'The AuthService')

Georgi-it · Accepted Answer · 2015-04-20T09:25:21.007

11

!! Converts any value to a boolean value

 > !!null
 false

 > !!true
 true

 > !!{}
 true

 > !!false
 false

If a value is falsey then the result will be false. If it is truthy the result will be true.

Even more, the third ! inverts the converted value so the above examples become:

    > !!!null
    true

    > !!!true
    false

    > !!!{}
    false

    > !!!false
    true

edited Apr 20 '15 at 09:25

answered Apr 20 '15 at 09:15

Georgi-it

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You won't ever need three `!`. Just one or two :) – Sergio Tulentsev Apr 20 '15 at 09:28
Yeah, but the OP was asking, so I just went to clarify the concept behind this :) – Georgi-it Apr 20 '15 at 09:30
yeah, but it came out confusing. The concept here is that the second `!` negates the first negation, thus restoring value to its original state of truthiness, but now as real boolean (which is enforced by the first `!`) – Sergio Tulentsev Apr 20 '15 at 09:32

score 2 · Answer 2 · answered Apr 20 '15 at 09:16

It forces what is returned to be a boolean and not an integer or empty value. For example, 0 evaluates to false with == but will not with ===. So to be sure that any integer 0 returned will be converted into an boolean, we use !!. This also works if null or undefined is returned.

So whats happening is actually:

var test = null;
var result = !test; // returns true
    result = !return; // returns false

score 1 · Answer 3 · answered Apr 20 '15 at 09:16

1

!! is used to convert the value to the right of it to its equivalent boolean value.

!!false === false
!!true === true

answered Apr 20 '15 at 09:16

Rahul Tripathi

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score 0 · Answer 4 · edited May 23 '17 at 11:43

0

Coerces oObject to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true.

!oObject  //Inverted boolean
!!oObject //Non inverted boolean so true boolean representation

So !! is not an operator, it's just the ! operator twice.

Referred from : https://stackoverflow.com/a/784946/2218635

edited May 23 '17 at 11:43

Community

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answered Apr 20 '15 at 09:16

Ramesh Rajendran

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Meaning of !! javascript

4 Answers4