> Unless what transpires is a "back link" to prior definitions such that additions to those prototypes are reflected in subsequent references. Quite frankly, I don't recall off the top of my head which way compliant EMCAScript implementations behave.

It is:

    var a = {}; var b = {__proto__: a}; console.log(b.x); a.x = 5; console.log(b.x);

(prints undefined then 5)