fake-class.js

const {keys, create, assign} = Object;

function fakeClass(Constructor, prototype) {
  const methods = keys(Constructor.prototype);
  const overrides = keys(prototype);
  methods.forEach(function (method) {
    if (!overrides.includes(method)) {
      throw new Error(`Method "${method}" missing in Overrides for class [${Constructor.name}]`);
    }
  });

  function FakeClass() {
    Constructor.apply(this, arguments);
  }
  FakeClass.prototype = assign(create(Constructor.prototype), prototype);

  return FakeClass;
}

usage.js

// class to be faked
function SomeClass() {}

SomeClass.prototype.foo = function () {
  console.log("foo");
};

SomeClass.prototype.bar = function () {
  console.log("bar");
};


// this throws because method 'bar' is missing
const FailingFake = fakeClass(SomeClass, {
  foo: function () {}
});

// create a fake class
const Fake = fakeClass(SomeClass, {
  // override 'foo' with own behaviour
  foo: function () {
    this._fooCalled = true;
  },

  // re-use 'bar' as is
  bar: SomeClass.prototype.bar,

  // add additional methods
  fooCalled() {
    return this._fooCalled;
  }
});


// create an instance
const someClassFake = new Fake();


someClassFake instanceof SomeClass; // => true
someClassFake instanceof Fake; // => true

someClassFake.bar(); // logs 'bar' to the console
someClassFake.foo(); // does not log to the console
someClassFake.fooCalled(); // => true