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Good afternoon in my timezone

I saw this line of code in a book

!!(document.all && document.uniqueID);

Why is necessary to use the double not operator ? Do not the && operator already transforms the result to a boolean?

Thanks in advance Best regards

tt0686
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    IMO !! is unnecessary here – webduvet Apr 01 '14 at 11:33
  • No, the && operator doesn't change the result to a boolean, and yes, it may be necessary, depending on whether you need or not a boolean. – Denys Séguret Apr 01 '14 at 11:35
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    @lombausch Why do you say that ? If the author wants a boolean, this may be necessary. – Denys Séguret Apr 01 '14 at 11:35
  • " *However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value* " : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators – Alex K. Apr 01 '14 at 11:36
  • @AlexK. :) I had no idea it works like this. thanks for pointing out. – webduvet Apr 01 '14 at 11:38

1 Answers1

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&& will return value of document.uniqueID (if document.all has any value other than '',false, null, undefined, 0, NaN) for document.all && document.uniqueID 

 for Example 45 === (true && 45) //true

From mozilla.org

Logical AND (&&) expr1 && expr2 Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.

So it is necessary to use !! to convert above expression into Boolean

Anoop
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