Accessing a layer in a precomp (syntax question)

Let's say your main comp is called "A", and in there you have a precomp "B". Inside "B" you've got a layer called "L" that you want to access.

To do that you need to get "B" as an AVLayer, then get it as a CompItem by using it's source:

var a_comp = app.project.activeItem;
if (a_comp && a_comp instanceof CompItem)
{
    var b_layer = a_comp.layer("B");
    var b_comp = b_layer.source;
    if (b_comp && b_comp instanceof CompItem)
    {
        var l = b_comp.layer("L");
     }
}

I added the checks that a_comp and b_comp exist and are indeed CompItem variables just as an example, you should obviously also check that b_layer isn't null.