Meaning of [-1,1][+!!boolean_var]

Question

Can someone please explain the purpose of double-negating the reverse var in the below code?

return function (a,b) {
   var A = key(a), B = key(b);
   return ((A < B) ? -1 :
           (A > B) ? +1 : 0)) * [-1,1][+!!reverse];                  
}

The way I understand it, the purpose is to pick the proper index from the [-1,1] array to then use it in the multiplication but it seems to me that [-1,1][+!!reverse]; could be safely replaced by [-1,1][+reverse];

Am I wrong? What do you gain or prevent by double-negating reverse there?

I saw the above code on this answer.

Answer 1

The easiest answer is probably a counter-example:

+undefined //NaN
+!!undefined // 0

since contents of [] are generally converted to strings, [NaN] will attempt to access the property called "NaN" from the array, which does not exist and will return undefined:

[1,-1][+undefined]
[1,-1][NaN]
[1,-1]["NaN"]
undefined

Answer 2

Double negating simply ensures that we have a proper boolean. Since reverse could be anything, double negating it invokes JavaScript's "falsy" conversions. So, for example:

!!"hello" // true
!!"" // false
!!1 // true
!!some_undefined_var // false

Answer 3

The !! converts the value to a boolean. This is needed if reverse isn't a boolean to start off with.

Consider: +!!"hi". This is 1, but +"hi" is NaN.