How to make a singleton promise.

asyncSingleton.js

// a connection singleton.
let dbConnectionPromise;
function getDb() {
  if (dbConnectionPromise) {
    return dbConnectionPromise;
  }
  dbConnectionPromise = DbClient.connect(options);
  return dbConnectionPromise;
}

// partition worker using singleton.
const writeSomeTheThings = async (someThings) => {
  const db = await getDb();
  await writeThings(db, someThings);
};

// parallel writer.
const writeAllOfTheThings = (allThings) =>
  Promise.all(getSome(allThings).map(writeSomeTheThings));