Determine if a user called "then" on registration promise.

spy.js

const spyInstance = (on, replace, spy) => {
  const _real = on[replace];
  on[replace] = function(...args) {
    const ret = _real.apply(this, args);
    const newRet = spy(args, ret, this);
    if (newRet) return newRet;
    return ret;
  };
};

const spy = (on, replace, spy) => {
  const _real = on.prototype[replace];
  on.prototype[replace] = function(...args) {
    const ret = _real.apply(this, args);
    const newRet = spy(args, ret, this);
    if (newRet) return newRet;
    return ret;
  };
};

spyInstance(console, "log", ([maybeRegistration]) => {
  if (maybeRegistration instanceof ServiceWorkerRegistration) {
    console.log("User logged registration!");
  }
});

spy(ServiceWorkerContainer, "register", (register, userReturned) => {
  console.log("User called register!");
  spy(Promise, "then", ([registration], returned, instance) => {
    if (userReturned === instance) {
      console.log("User called then on promise returned by register!");
    }
  });
});