By doing this you evaluate the regular expression every time you call the method.

You should choose between two options:

1. Do everything in the constructor
2. Do as you did now, but rewrite the function

````js
isDevice : function () {
   // whatever logic you like
   if (previousComputationIsTrue) {
      this.isDevice = Aria.returnTrue;
   } else {
      this.isDevice = Aria.returnFalse;
   }
   return this.isDevice();
}
````
By doing this you'll execute the regexp only once.

Moreover, never call an internal variable `_something`, the underscore is just useless and make things less readable.

It would be nice to have a performance test to understand whether there is a loss doing everything in the constructor. The drawback of solution (2) is extra code -> bigger file.