PS scripting in JS. How to acquire the Content Layer of a Frame Layer's?

Further to my previous post.

Assuming that the frame layer is active and that it has content, selecting the top frame layer's content by index number:

app.activeDocument.activeLayer = app.activeDocument.activeLayer.layers[0];

.

 Or using the first layerSet index number in the collection (you obviously need to know which layerSet index number to target, as opposed to the entire layers collection):

app.activeDocument.activeLayer = app.activeDocument.layerSets[0].layers[0];

Or just using the layers absolute position in the stack (not isolating by layerSet), such as the very back layer index:

app.activeDocument.activeLayer = app.activeDocument.layers[app.activeDocument.layers.length - 1].layers[0];

I haven't tried replacing content, hopefully the same principle applies.