How does !!~ (not not tilde/bang bang tilde) alter

Question

If you read the comments at the jQuery inArray page here, there's an interesting declaration:

!!~jQuery.inArray(elm, arr) 

Now, I believe a double-exclamation point will convert the result to type boolean, with the value of true. What I don't understand is what is the use of the tilde (~) operator in all of this?

var arr = ["one", "two", "three"];
if (jQuery.inArray("one", arr) > -1) { alert("Found"); }

Refactoring the if statement:

if (!!~jQuery.inArray("one", arr)) { alert("Found"); }

Breakdown:

jQuery.inArray("one", arr)     // 0
~jQuery.inArray("one", arr)    // -1 (why?)
!~jQuery.inArray("one", arr)   // false
!!~jQuery.inArray("one", arr)  // true

I also noticed that if I put the tilde in front, the result is -2.

~!!~jQuery.inArray("one", arr) // -2

I don't understand the purpose of the tilde here. Can someone please explain it or point me towards a resource?

Answer 1

There's a specfic reason you'll sometimes see ~ applied in front of $.inArray.

Basically,

~$.inArray("foo", bar)

is a shorter way to do

$.inArray("foo", bar) !== -1

$.inArray returns the index of the item in the array if the first argument is found, and it returns -1 if its not found. This means that if you're looking for a boolean of "is this value in the array?", you can't do a boolean comparison, since -1 is a truthy value, and when $.inArray returns 0 (a falsy value), it means its actually found in the first element of the array.

Applying the ~ bitwise operator causes -1 to become 0, and causes 0 to become `-1. Thus, not finding the value in the array and applying the bitwise NOT results in a falsy value (0), and all other values will return non-0 numbers, and will represent a truthy result.

if (~$.inArray("foo", ["foo",2,3])) {
    // Will run
}

And it'll work as intended.

Answer 2

jQuery.inArray() returns -1 for "not found", whose complement (~) is 0. Thus, ~jQuery.inArray() returns a falsy value (0) for "not found", and a truthy value (a negative integer) for "found". !! will then formalise the falsy/truthy into real boolean false/true. So, !!~jQuery.inArray() will give true for "found" and false for "not found".

Answer 3

The ~ for all 4 bytes int is equal to this formula -(N+1)

SO

~0   = -(0+1)   // -1
~35  = -(35+1)  // -36 
~-35 = -(-35+1) //34 

Answer 4

I assume, since it is a bitwise operation, it is the fastest (computationally cheap) way to check whether path appears in modifiedPaths.

Answer 5

You're right: This code will return false when the indexOf call returns -1; otherwise true.

As you say, it would be much more sensible to use something like

return this.modifiedPaths.indexOf(path) !== -1;

Answer 6

The tilde operator isn't actually part of jQuery at all - it's a bitwise NOT operator in JavaScript itself.

See The Great Mystery of the Tilde(~).

You are getting strange numbers in your experiments because you are performing a bitwise logical operation on an integer (which, for all I know, may be stored as two's complement or something like that...)

Two's complement explains how to represent a number in binary. I think I was right.