Joos is a *short form*, not a diminutive - if it were, it would have been something along the lines of Joosje, Jooske, etc. [noted -ed]Also, apart from what is already listed in the database entry, Joos is also a variant form of Joas in The Netherlands.
Also, apart from what is already listed in the database entry, Joos is also a variant form of Joas in The Netherlands.