resolve promise sequentially by reduce

reduce-promises.js

[...cohortSlugs].reduce((prevPromise, slug) => {
  let applications;
  return prevPromise
    .then((acc) => {
      applications = acc;
      return Cohort.findOne({ slug });
    })
    .then(({ _id: cohort }) => [
      ...applications,
      ...applicationsFromFirestore
        .filter(({ cohortId }) => cohortId === slug)
        .map(({ cohortId, ...rest }) => ({ cohort, ...rest })),
    ]);
}, Promise.resolve([]));