CachedPhones.js

const cache = {};

function changePhonesStatus(phones = {}, unavailable = []) {
  const key = JSON.stringify(unavailable);
  // generates a string array e.g. "[1,2,3]"
  if (cache[key]) {
    return cache[key];
  }
  // we need to look at each department and update the phones for that department. 
  const updatedPhones = Object.keys(phones.departments).reduce((acc, key) => {
    const departmentPhones = phones.departments[key];
    return [
      ...acc,
      ...departmentPhones.map((phone) => ({
        ...phone,
        status: unavailable.includes(phone.id) ? "IN_CALL" : "AVAILABLE"
      }))
    ];
  }, []);
  // once we've updated it we want to ensure we set that in our cache for later use
  cache[key] = updatedPhones;
  return cache[key];
}