Is like getNum but instead of asserting if Object is not a num it
returns the given default value

I find it much easier to read
    rect->x1 = obj1.arrayGet(0).getNumWithDefaultValue(0);
than
    (obj2 = obj1.arrayGet(0), obj2.isNum() ? rect->x1 = obj2.getNum() : rect->x1 = 0);

On top of it has the benefit of being slightly faster