How do you implement a “raceToSuccess” helper, given a list of promises?

This is a classic example where inverting your logic makes it much clearer. Your “race” in this case is that you want your rejection behavior to in fact be success behavior.

function oneSuccess(promises){
  return Promise.all(promises.map(p => {
    // If a request fails, count that as a resolution so it will keep
    // waiting for other possible successes. If a request succeeds,
    // treat it as a rejection so Promise.all immediately bails out.
    return p.then(
      val => Promise.reject(val),
      err => Promise.resolve(err)
    );
  })).then(
    // If '.all' resolved, we've just got an array of errors.
    errors => Promise.reject(errors),
    // If '.all' rejected, we've got the result we wanted.
    val => Promise.resolve(val)
  );
}