Meaning of !! javascript [duplicate]

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')

Answer 1

!! 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

Answer 2

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

Answer 3

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

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