why use not not !! for boolean value in MS AJAX?

Question

From MS AJAX source code,

Type.isClass = function Type$isClass(type) {
/// <summary locid="M:J#Type.isClass" />
/// <param name="type" mayBeNull="true"></param>
/// <returns type="Boolean"></returns>
var e = Function._validateParams(arguments, [
    {name: "type", mayBeNull: true}
]);
if (e) throw e;
if ((typeof(type) === 'undefined') || (type === null)) return false;
return !!type.__class;

}

Please look at the last line of code, why use !! instead of just return type.__class?

Thanks!

Answer 1

Guaranteed safe boolean conversion.

Answer 2

The author must not have thought type.__class was guaranteed to be a boolean value. Since you can pass any object to Type.isClass():

Type.isClass(3);
Type.isClass({});
Type.isClass(AnActualClassFunction);

...there's really no guarantee that type.__class will have a boolean value.

Of course, parameters that aren't ASP.NET AJAX "classes" generally won't have a __class property - so if the routine simply returned type.__class, the possible return values would be true, false, and undefined. This is undesirable for a routine intended to return a boolean.

!!type.__class just guarantees that a boolean value (true or false) is returned (undefined will be converted to false).

Answer 3

So basically there is no difference between !!val and Boolean(val)

var val; // val is undefined
alert(!!val); // display false
alert(Boolean(val)); // display false as well