I get promises, but in this example:

function getFullName( id ){ return ajax( "/user/" + id + "/name" ) + " " + ajax( "/user/" + id + "/lastname" ) }

How does it know to to wait for both ajax functions before returning the correct value? Am I misunderstanding something?

See my response to virulent. I believe it re-runs the parent function multiple times until all of the child ajax functions have evaluated to their return values. This probably explains why the author states that the child functions must be idempotent (edit: and the code readonly in its entirety to prevent side effects).