43.groupBirthdays

Write a function named groupBirthdays that receives 2 parameters:

• an Array of users each with 2 properties:
  • name - a string representing the user's name
  • birthday - a Date representing the user's birthday
• a groupBy string with one of the following values - month, day, year

The function should group the users according to their birthdays and the groupBy parameter, and return a Map where the values are Arrays of users, and the keys are:

• index of the month if groupBy="month" (starting from 1)
• index of the day in a month if groupBy="day" (starting from 1)
• year if the grouping string is groupBy="year"