js syntax : how to interpret this : return!!(exp) ;

Question

working on a page basic example, I found this piece of code :

function hasGetUserMedia() {

return !!(navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia);

} 

and been astonished by : return!!(exp)

Can someone explain this '!!' ? is it for the line break ? or to avoid a return value ?

I could not find usage nor info about this kind of (weird) syntax

thanks !

Answer 1

The !! coerces the value into a boolean that represents whether the original value is "truthy" or "falsy". For example:

!!"foo" // true
!!""    // false

Given that ! is the negation operator, using it once will convert a value into a boolean that is the opposite of it's truthy/falsy value. E.g.,

!"foo" // false
!""    // true

Adding another ! negates the negation, resulting in a boolean that matches the truthy/falsy-ness of the original non-boolean value.

Answer 2

The code below forces the expression to be converted to boolean.

function hasGetUserMedia() {

    return !!(navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia);

}