Make a Marquee Selection

I want the variable ArtSet to "selectAll", which would simple just be selecting the entire canvas a rectangular marquee.  What am I doing wrong ?

app.documents.add(600,200)

var layerRef = app.activeDocument.artLayers.add()

layerRef.name  = "mylayer"

layerRef.blendMode = BlendMode.OVERLAY

var artset = activeDocument.artLayers.add()

artset.name = "goat"

artset.blendMode = BlendMode.SCREEN

// I want the variable artset to be selected or selected all that is not happening ?

artset.activeDocument.selection.selectAll