[Typescript] Enum: Numeric Enums and Reverse Mappings

Let's say we have enum defined as such:

export enum WeekDayEnum {
  sunday = 0,
  monday,
  tuseday,
  wednesday,
  thursday,
  friday,
  saturday,
}
export type DayOfWeek =
  | "monday"
  | "tuseday"
  | "wednesday"
  | "thursday"
  | "friday"
  | "saturday"
  | "sunday";

 

Usage 1: pass number value as lookup for string key

export const getToday = (_date?: Date) => {
  const date: Date = _date || new Date();
  const year = date.getFullYear();
  const month = date.getMonth();
  const day = date.getDate();
  return new Date(year, month, day);
};
export const isSameWeekDay = (day: DayOfWeek, _date?: Date) => {
  const today = getToday(_date).getDay();
  return WeekDayEnum[today] === day;
};

'today' is a number type, so :

WeekDayEnum[today] // 'monday'

 

 

usage 2: pass string key to lookup number value

const getTodayOpeningTime = (
  timeTable: WeeklyTimeTableVM[]
): WeeklyTimeTableVM => {
  const todayNum = getToday().getDay() as weeksNumber;
  const todayStr = WeekDayEnum[todayNum];
  return timeTable.find((day) => day[0] === todayStr) as WeeklyTimeTableVM;
};