need help understanding some JS code

I am reading through some JS code for DHTML menus

here is how it begins

ypSlideOutMenu.Registry = []
ypSlideOutMenu.aniLen = 250
ypSlideOutMenu.hideDelay = 1000
ypSlideOutMenu.minCPUResolution = 10

// constructor
function ypSlideOutMenu(id, dir, left, top, width, height)
{
this.ie = document.all ? 1 : 0
this.ns4 = document.layers ? 1 : 0
this.dom = document.getElementById ? 1 : 0

what is the "this.ie", "this.ns4", "this.dom"?
are they some special variables?

the complete code is here:
http://www.blender3d.org/_js/ypSlideOutMenus.js

the web page is
http://www.blender3d.org/

test wrote:
[...]

what is the "this.ie", "this.ns4", "this.dom"?
are they some special variables?
[...]

No. They are just some variables that are set to '1' or '0' depending
upon whether the browser the script is running in supports the methods
document.all, document.layers or document.getElementById. They should
probably be set to true or false, but 1 and 0 have the same effect.

The implication is that browsers supporting document.all are "IE",
those that support document.layers are "Netscape 4" and those that
support document.getElementById are DOM compliant.

Of course, some browsers support more than one, but I guess that
doesn't really matter.

Rob.