Examples of TypeScript's powerful type system

enum-composition.ts

enum Weekday {
  MONDAY = 'MONDAY',
  TUESDAY = 'TUESDAY',
  WEDNESDAY = 'WEDNESDAY',
  THURSDAY = 'THURSDAY',
  FRIDAY = 'FRIDAY',
}

enum Weekend {
  SATURDAY = 'SATURDAY',
  SUNDAY = 'SUNDAY',
}

enum Day {
  ...Weekday,
  ...Weekend,
}
type Day = Weekday | Weekend;


// Usage Example

// see enum-map.ts below first
type EnumMap<E, V> = {
  [K in keyof E]: V
};

const daysToLabels: EnumMap<Day, string> = {
  MONDAY: 'Monday',
  TUESDAY: 'Tuesday',
  WEDNESDAY: 'Wednesday',
  THURSDAY: 'Thursday',
  FRIDAY: 'Friday',
  SATURDAY: 'Saturday',
  SUNDAY: 'Sunday',
};

function tgif(day: Day) {
  if (input === Weekday.FRIDAY) {
    alert('Thank God');
  }

  alert(`It's ${daysToLabels[day]}!`);
}

tgif(Day.FRIDAY); // Thank God It's Friday!

const sunday: Day = Weekend.SUNDAY;
tgif(sunday); // It's Sunday!

enum-map.ts

// Note that this only works with String enums.
// If you want to use it with a Numeric enum or Heterogenous enum, convert it to a String enum, then move the values to an EnumMap<E, number> or EnumMap<E, any> respectively.
type EnumMap<E, V> = {
  [K in keyof E]: V
};


// Usage Example

enum Direction {
  NORTH = 'NORTH',
  SOUTH = 'SOUTH',
  EAST = 'EAST',
  WEST = 'WEST',
}

const directionsToAbbreviations: EnumMap<typeof Direction, string> = {
  // unlike using a Map<Direction, string>, this is exhaustive. If any Enum value is not mapped, it will not compile.
  NORTH: 'N',
  SOUT: 'S',
  EAST: 'E',
  WEST: 'W',
};

alert(directionsToAbbreviations[Direction.NORTH]);  // N

rest-param.ts

// Allows usage of Rest Parameters in type parameters
export type RestParam<T> = T | T[];

export function restParamToArray<T>(restParam: RestParam<T>): T[] {
  return Array.isArray(restParam) ? restParam : [restParam];
}


// Usage Example

enum Role {
  USER,
  ADMIN,
}

enum Permission {
  READ,
  WRITE,
}

class Authorizer {
  protected rolesToPermissions: Map<Role, Set<Permission>>;

  constructor(rolesToPermissions: Map<Role, RestParam<Permission>>) {
    this.rolesToPermissions = new Map(
      Array.from(rolesToPermissions, ([role, permissions]) =>
        [role, new Set(restParamToArray(permissions))]
      )
    );
  }

  roleHasPermission(role: Role, permission: Permission): boolean {
    const permissions = rolesToPermissions.get(role);
    return permissions?.has(permission);
  }
}

const authorizer = new Authorizer(new Map<Role, RestParam<Permission>>([
  [Role.USER, Permission.READ],
  [Role.ADMIN, [
    Permission.READ,
    Permission.WRITE,
  ]],
]));

alert(authorizer.roleHasPermission(Role.USER, Permission.WRITE)); // false
alert(authorizer.roleHasPermission(Role.ADMIN, Permission.WRITE));  // true