Enums and Advanced Types

Enums

An enum gives a set of related constants readable names.

TypeScript

enum Role {
  Student = 'STUDENT',
  Instructor = 'INSTRUCTOR',
  Admin = 'ADMIN',
}

function permissions(role: Role): string[] {
  switch (role) {
    case Role.Admin:
      return ['read', 'write', 'delete'];
    case Role.Instructor:
      return ['read', 'write'];
    default:
      return ['read'];
  }
}

console.log(permissions(Role.Instructor));

enum Role {
  Student = 'STUDENT',
  Instructor = 'INSTRUCTOR',
  Admin = 'ADMIN',
}

function permissions(role: Role): string[] {
  switch (role) {
    case Role.Admin:
      return ['read', 'write', 'delete'];
    case Role.Instructor:
      return ['read', 'write'];
    default:
      return ['read'];
  }
}

console.log(permissions(Role.Instructor));

Discriminated unions

Combining a union type with a shared "tag" property lets TypeScript narrow which shape you're working with.

TypeScript

type ApiResult<T> = { ok: true; data: T } | { ok: false; error: string };

function unwrap<T>(result: ApiResult<T>): T {
  if (!result.ok) throw new Error(result.error);
  return result.data; // TypeScript knows result is the { ok: true } branch here
}

console.log(unwrap({ ok: true, data: 42 }));

type ApiResult<T> = { ok: true; data: T } | { ok: false; error: string };

function unwrap<T>(result: ApiResult<T>): T {
  if (!result.ok) throw new Error(result.error);
  return result.data; // TypeScript knows result is the { ok: true } branch here
}

console.log(unwrap({ ok: true, data: 42 }));

The ok property is the discriminant, checking it narrows result to exactly one branch of the union inside each if.

Enums and Advanced Types

Enums

Discriminated unions

📝 Enums & Unions Quiz