In Windows: to create a button in the taskbar/hide a window; you need moderate-to-extensive access to the Windows API or lower. Javascript is a high level (and highly restricted) language designed to be deployed through browsers (which may or may not be running on Windows). Naturally; for too many reasons to list here; you cannot get that level of access from Javascript. If you could; I wouldn't leave Javascript turned on. On any other OS; the same hopefully applies.

You could do this if you were writing your own standalone application or browser plugin. But not from any webpage; and not with javascript.