pimpl.js

function NotPimpl(x) {
  // more than one assignment, "deep" copy
  this.first = x.first
  this.last = x.last
}

NotPimpl.prototype.fullName = function() {
  // direct members, fast
  return this.first + this.last
}

function Pimpl(x) {
  // one assignment, "shallow" copy
  this.x = x
}

Pimpl.prototype.fullName = function() {
  // indirect members, slower
  return this.x.first + this.x.last
}

x = {
  "first": "John",
  "last": "Doe"
}

np = new NotPimpl(x) // slower
np.fullName() // fast

p = new Pimpl(x) // fast
p.fullName() // slower

// altogether, the speed differences are peanuts not worth worrying about until you reach millions of objects.