Adobe Extending Flash Professional CS4 - Manual (Page 457)

435 EXTENDING FLASH CS4 PROFESSIONAL
SymbolItem object
Description
Read-only property; a
Timeline object .
Example
The following example obtains and shows the number of layers that the selected movie clip in the library contains:
var tl = fl.getDocumentDOM().library.getSelectedItems()[0].timeline;
alert(tl.layerCount);