Using double negation - For what reason?

Question

Yesterday it have read this blog-article by John Resig: http://ejohn.org/blog/comparing-document-position/

This snippet I don't understand:

function contains(a, b){
  return a.contains ?
    a != b && a.contains(b) :
    !!(a.compareDocumentPosition(b) & 16);
}

The first expression is clear: If a isn't identical with b and b is contained in a then return true. But what is the purpose of the two-times negation in the second expression? The return of compareDocumentPosition() is bitwise compared with 16. => Results either in 16 or 0. Let's say it is 16 then it becomes false after the first negation. Then true again after the second negation. In case of 0 it becomes true and then false again.

Does anyone understand the code and can explain it?

Okay. Makes sense. I guess I've been stucked. Thanks. Do you want to turn your comment into an answer so that I can mark it as accepted? — michael.zech, Jul 19 '15 at 07:39

score 1 · Accepted Answer · edited May 23 '17 at 12:29

1

The double negation operator is used to convert as boolean.. think of it as a similar method to parseInt()

Here's a related stackoverflow question

edited May 23 '17 at 12:29

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answered Jul 19 '15 at 07:36

Jad Joubran

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Using double negation - For what reason?

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