How do I interpolate a variable as a key in a JavaScript object?

How can I use the value of the variable a as a key to lookup a property? I want to be able to say: b["whatever"] and have this return 20:

var a = "whatever";
var b = {a : 20};     // Want this to assign b.whatever
alert(b["whatever"]); // so that this shows 20, not `undefined`

I am asking if it's possible during the creation of b, to have it contain "whatever":20 instead of a:20 where "whatever" is itself in a variable. Maybe an eval could be used?