How do I convert an attrs property to a boolean in angular?

Question

How do I convert attrs.requireItems to a boolean?

attrs.$observe("requireItems", function(){
    var val = attrs.requireItems;
    ngModel.$setValidity("require-items", !!val);
});

The possible values are all strings but can be "true" "false" "0" or "N" (where N is the length of an array).

The problem is when its "0" or "false" it returns true because its a string.