Get return type of interface function #16372

const tokens = {
  // key below is name of token
  month: {
    possibleValues: ['jan', 'feb'], // etc...
    filter: (monthName: string): number => {
      return 12; // will return number 1-12
    }, 
  },
  weekday: {
    possibleValues: ['mon', 'tue'], // etc
    filter: (monthName: string): Date => {
      return new Date(); // will return Date of weekday
    },
  },
};

// as seen above - filter of 'month' always returns number and filter of 'weekday' returns Date

const parser: Parser<typeof tokens> {
  parseWeekday: ({ weekday, month }) => {
    // weekday should be detected to be Date type
    // month should be detected to be number type
  }
}