enum Day {
    Monday = 'MON',
    Tuesday = 'TUE',
    Wednesday = 'WED',
    Thursday = 'THU',
    Friday = 'FRI'
}
enum Shift {
    Morning = 9,
    Afternoon = 13,
    Evening = 17,
    Night = 20
}
function employeeWorked(day: Day, shift: Shift) {
    // ...
}