Research about instance constants

instance-const.js

function Obj(x) {
  if (typeof x !== 'number') throw new Error()
  Object.defineProperty(this, 'x', {value: x})
}

var a = new Obj(1)
var b = new Obj(2)

// Do `a` and `b` have the same hidden class in v8?
//
// Does the above definition impact performances?
//
// From a performance point of view, is it worth 
// locking (using `defineProperty`) prototype's defs?

1. Yes, they have the same hidden class.
2. No.
3. Yes, for constants on prototype objects, except functions - those are already treated in a special way. V8 does not use it as much as it could, but I would expect it to start using it more and more in the future.