Map an object with Object.keys and Array.prototype.reduce

mapObject.js

const weekDays = {
  monday: "Monday",
  tuesday: "Tuesday",
  wednesday: "Wednesday",
  thursday: "Thursday",
  friday: "Friday"
};

const mappedWeekDays = Object.keys(weekDays).reduce((mapper, day) => {
  mapper[day] = {
    morning: true,
    afternoon: false
  };
  return mapper;
}, {});

console.log(mappedWeekDays);
// => { monday: { morning: true, afternoon: false }, tuesday: { morning: true, afternoon: false }, ... }