Enums and const assertions: the modern alternatives

Understand the limits of enums and derive your types from frozen constants with as const.

7 min · 3 questions

enum has been part of the language since its early days, and you'll run into it in plenty of codebases. It groups named constants under a single type, usable both as a value and as an annotation.

enum Role {
  User = "USER",
  Admin = "ADMIN",
}

function canDelete(role: Role): boolean {
  return role === Role.Admin;
}

Yet the modern ecosystem is moving away from it. An enum is syntax that doesn't exist in JavaScript: the compiler emits an object at runtime, which complicates certain tools and is incompatible with the erasableSyntaxOnly option. The alternative: a plain object frozen with as const, from which you derive the type.

const ROLES = {
  User: "USER",
  Admin: "ADMIN",
} as const;

type Role = (typeof ROLES)[keyof typeof ROLES];

function canDelete(role: Role): boolean {
  return role === ROLES.Admin;
}

as const also works on arrays: each element becomes a read-only literal, and (typeof SIZES)[number] produces the union of the values. A single source of truth feeds both the running code and the types.

const SIZES = ["sm", "md", "lg"] as const;

type Size = (typeof SIZES)[number];

function isSize(value: string): value is Size {
  return (SIZES as readonly string[]).includes(value);
}

Knowledge check

Make sure you remember the key points of this lesson.

1. What major difference separates an enum from an object frozen with as const?
   • An enum is TypeScript-specific syntax that generates code, while an as const object remains plain JavaScript
   • An as const object is slower at runtime
   • An enum can only contain numbers
2. What does as const produce when applied to the array ['sm', 'md']?
   • string[]
   • A read-only tuple of literal types
   • A mutable array of type Array<'sm' | 'md'>
   • A compile error
3. How do you get the union of the values of a ROLES object frozen with as const?
   • keyof typeof ROLES
   • (typeof ROLES)[keyof typeof ROLES]
   • typeof ROLES