How can i get the id of the canvas?

HI,thans you help first!

for example:

lib.properties = {  id: '054D1722FC18B043A207F8FDCA4D6A20',  width: 1024,  height: 768,  fps: 12,  color: "#FFFFFF",  opacity: 1.00,  manifest: [  {src:"images/_1.jpg?1498542790116", id:"_1"}  ],  preloads: []
};

we can get the lib like this:

var comp=AdobeAn.getComposition("054D1722FC18B043A207F8FDCA4D6A20");
var lib=comp.getLibrary();

but ,if i don't know the id,so i can't get the id like above.

so how can it do it ?

thanks !