Prototype Library use of !! operator [duplicate]

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Possible Duplicates:

What does this expression mean “!!”

What does the !! operator 开发者_开发技巧(double exclamation point) mean in JavaScript?

Here is a snippet from Prototype Javascript Library :

  Browser: (function(){
    var ua = navigator.userAgent;
    var isOpera = Object.prototype.toString.call(window.opera) == '[object Opera]';
    return {
      IE:             !!window.attachEvent && !isOpera,
      Opera:          isOpera,
      WebKit:         ua.indexOf('AppleWebKit/') > -1,
      Gecko:          ua.indexOf('Gecko') > -1 && ua.indexOf('KHTML') === -1,
      MobileSafari:   /Apple.*Mobile/.test(ua)
    }
  })(),

This is all good and i understand the objective of creating a browser object. One thing that caught my eye and I haven't been able to figure out is the use of double not operator !! in the IE property.

If you read through the code you will find it at many other places. I dont understand whats the difference between !!window.attachEvent and using just window.attachEvent.

Is it just a convention or is there more to it that's not obvious?

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I dont understand whats the difference between !!window.attachEvent and using just window.attachEvent.

The key of understanding this, is to know that the Boolean Logical Operators can return an operand, and not a Boolean result necessarily:

The Logical AND operator (&&), will return the value of the second operand if the first is truly:

true && "foo"; // "foo"

And it will return the value of the first operand if it is by itself falsy:

undefined && "anything"; // undefined
NaN && "anything";       // NaN
0 && "anything";         // 0

So, in the snippet !!window.attachEvent && !isOpera, we already know that isOpera is a boolean value, !! will just make sure that Browser.IE is a boolean result also.

An example: let's say we are in Firefox, window.attachEvent is undefined and !isOpera is true, if you don't use the double negation, Browser.IE would be undefined instead false.

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It's a fairly common trick used to coerce a value to a boolean type, instead of using (bool). !window.attachEvent would negate the truth value of window.attachEvent, giving you a boolean; !!window.attachEvent negates that, giving you the original truth value but as a boolean instead of the type of window.attachEvent