loader.js

const getManyUsers = async (ids) => {
  const USERS = {
    1: { name: "foo" },
    2: { name: "bar" }
  };
  return ids.map(id => USERS[id] || null);
}

function createLoader(getMany) {

  return async function (key) {
    const res = await getMany([key]);
    return res[0];
  };
}

function createLoader2(getMany) {
  const buf = [];
  return function (key) {
    return new Promise((resolve, reject) => {
      buf.push(key);
      process.nextTick(() => {
        const many = await getMany(buf);
        resolve(many[key]);
      })
    });
  };


  const getOneUser = createLoader2(getManyUsers);

  (async function main() {
    console.log(await getOneUser(1));
    console.log(await getOneUser(4));
  })();