Adobe Extending Flash Professional CS5 - Manual (Page 456)

434 EXTENDING FLASH PROFESSIONAL
SymbolItem object
Last updated 5/2/2011
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);