!undefined = true

!!undefined = false
!!!undefined = true

!false = true
!!false = false
!!!false = true

!true = false
!!true = true
!!!true = false

when using isEmailAvailable (positive) in correlation with exists (positive) make sure that the outcome is a positive return (and not an inverted)

src/user.js

@@ -276,7 +276,7 @@ var utils = require('./../public/src/utils.js'),
 
 	User.isEmailAvailable = function(email, callback) {
 		RDB.exists('email:' + email + ':uid' , function(err, exists) {
-			callback(err, !!exists);
+			callback(err, !!!exists);
 		});
 	}