Return the day of the week given an offset in days

dayOffset.js

// dayOffset("Wednesday", 4) 
// => "Sunday"

const dayOffset = (day, offset) => {
  const days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];
  
  if (days.indexOf(day) === -1) {
    return "😭 Not a valid day, dummy!";
  }
  
  return days[(days.indexOf(day) + offset) % 7];
}