minimal observable.js

function Ohm(v, i, _){
  (_=(k, get, set) => Object.defineProperty(this, k, {get, set, enumerable: 1}) && _)
   ("v", _=>v,   x=>v=x)
   ("i", _=>i,   x=>i=x)
   ("w", _=>v*i, x=>i=x/v)
   ("r", _=>v/i, x=>i=v/x);
}




var x = new Ohm(5, 2);          // == {"v":5,"i":2,"w":10,"r":2.5}
x.r=10;
console.log(JSON.stringify(x)); // == {"v":5,"i":0.5,"w":2.5,"r":10}