How To Create An Arbitrary Object At Runtime?

Can anyone suggest how to create an arbitrary object at runtime
WITHOUT using the deprecated eval() function. The eval() method works
ok (see below), but is not ideal.

function Client() { }
Client.prototype.fullname = "John Smith";
var s = "Client";
eval("var o = new " + s + "();");
alert(o.fullname);

Note: I want the type name of the object to be represented as a string
(in this case "Client" -- this is a non-negotiable requirement).

I also tried the following (which appears to fail):

function Client() { }
Client.prototype.fullname = "John Smith";
var s = "Client";
var o = new Object();
o.construct = s;
alert(o.fullname);

eval() is handy but not future-proof, so any suggestions would be
welcome.