How to convert all shapes in a AN file to bitmap using jsfl?

loop through all timelines, layers and keyframes selecting each element and then test for a shape and, if true, convert to bitmap:

var sel = fl.getDocumentDOM().selection[0];  // this is going to be the (possibly) time-consuming part 
if(sel.elementType == "shape")
fl.getDocumentDOM().convertSelectionToBitmap()
}