function Parent(x) { this.x = x }

Parent.prototype.showX = function () { console.log(this.x) }

function Child(x, y) { Parent.call(this, x); this.y = y; }

Child.prototype = Object.create(Parent.prototype);
Child.prototype.showXY = function () { console.log(this.x, this.y) }

var tests = [
  function shouldInitializeParentInstanceFields(Parent, Child) {
    var c = new Child(1, 2);
    return c.x === 1;
  },
  function shouldInitializeChildInstanceFields(Parent, Child) { /* ... */ },
  function childIsInstanceOfParent(Parent, Child) { /* ... */ },
  function childIsInstanceOfChild(Parent, Child) { /* ... */ },
  function parentIsInstanceOfParent(Parent, Child) { /* ... */ },
  function parentIsNotInstanceOfChild(Parent, Child) { /* ... */ },
  function parentMethodsAreAvailableInParent(Parent, Child) { /* ... */ },
  function parentMethodsAreAvailableInChild(Parent, Child) { /* ... */ },
  function childMethodsAreNotAvailableInParent(Parent, Child) { /* ... */ },
  function childMethodsAreAvailableInChild(Parent, Child) { /* ... */ },
  function dynamicallyAddedParentMethodIsAvailableInParent(Parent, Child) { /* ... */ },
  function dynamicallyAddedParentMethodIsAvailableInChild(Parent, Child) { /* ... */ },
  function dynamicallyAddedChildMethodIsNotAvailableInParent(Parent, Child) { /* ... */ },
  function dynamicallyAddedChildMethodIsAvailableInChild(Parent, Child) { /* ... */ },
  function deletedParentInstancePropertyDisappearsFromChild(Parent, Child) { /* ... */ },
]

for (var test of tests) {
  console.log(test(Parent, Child), test.name)
}