Document.all Vs. Document.layers

As I understand it, document.layers only works in Netscape 4+ (ie. it
doesn't work in any version of Internet Explorer), while document.all
only works in Internet Explorer 4+ (ie. it doesn't work in any version
of Netscape). Is that correct?

Also, what about document.getElementById? When did Internet Explorer
start supporting that and when did Netscape start supporting that?

I ask because I'm wondering how necessary doing something similar to
the following is:

if(document.getElementById)
obj = document.getElementById(id);
else if(document.all)
obj = document.all[id];
else if(document.layers)
obj = document.layers[id];
else
return 1;