javascript - Meaning of [-1,1][+!!boolean_var] - Stack Overflow

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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.

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.

• javascript

Share Improve this question Follow edited May 23, 2017 at 12:20 CommunityBot 111 silver badge asked Mar 30, 2012 at 20:21 IcarusIcarus 64k1414 gold badges101101 silver badges116116 bronze badges 4

• That is awesome, curious to see an answer on this one – Dave Thomas Commented Mar 30, 2012 at 20:23
• It changes the value of reverse to a boolean. What is the !! (not not) operator in JavaScript? – Mizuho Commented Mar 30, 2012 at 20:27
• 1 I would use (reverse? 1 : -1) instead of [-1,1][+!!reverse], IMO is just more clear, it's cheaper, it doesn't need an array object and accessing an index, and even the boolean conversion is implicitly made by the conditional operator... – Christian C. Salvadó Commented Mar 31, 2012 at 4:56
• @CMS I agree, if the whole purpose is just to multiply by 1 when true; otherwise by -1, reverse?1:-1 is clearer and cheaper. – Icarus Commented Mar 31, 2012 at 5:06

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4 Answers 4

Sorted by: Reset to default 5

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

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

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.

the double-negation basically casts reverse as a boolean, and then + will cast that boolean as an int.

so, if reverse is possibly: "yes", "no", "", true, false, 0,1,2, undefined

the double negation will make "yes" -> true "no" -> true "" -> false true -> true false -> false undefined -> false

then, the + will convert trues to 1, and falses to 0, so that it can be used as an array index.

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