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Avoid TypeScript Enums

TypeScript enums are not fully type-safe and can cause surprises. Your code should use constant objects instead of introducing a new enum.

  • Use the same name for your type- and value-declaration.
  • Use type to derive type information from the const object.
  • Avoid asserting the type of an enum-like. Use explicit types instead.
  • Create utilities to convert and identify enums modelled as primitives.

Numeric enum-likes​

Given the following enum:

export enum CipherType {
  Login: 1,
  SecureNote: 2,
  Card: 3,
  Identity: 4,
  SshKey: 5,
};

You can redefine it as an object like so:

// freeze to prevent member injection
export const CipherType = Object.freeze({
  Login: 1,
  SecureNote: 2,
  Card: 3,
  Identity: 4,
  SshKey: 5,
} as const);

// derive the enum-like type from the raw data
export type CipherType = (typeof CipherType)[keyof typeof CipherType];

And use it like so:

// Can be imported together
import { CipherType } from "./cipher-type";

// Used as a type
function doSomething(type: CipherType) {}

// And used as a value (just like a regular `enum`)
doSomething(CipherType.Card);

// advanced use-case: discriminated union definition
type CipherContent =
  | { type: typeof CipherType.Login, username: EncString, ... }
  | { type: typeof CipherType.SecureNote, note: EncString, ... }
warning

Unlike an enum, TypeScript lifts the type of the members of const CipherType to number. Code like the following requires you explicitly type your variables:

// ✅ Do: strongly type enum-likes
let value: CipherType = CipherType.Login;
const array: CipherType[] = [CipherType.Login];
const subject = new Subject<CipherType>();

// ❌ Do not: use type inference
let value = CipherType.Login; // infers `1`
const array = [CipherType.Login]; // infers `number[]`

// ❌ Do not: use type assertions
let value = CipherType.Login as CipherType; // this operation is unsafe

String enum-likes​

The above pattern also works with string-typed enum members:

// freeze to prevent member injection
export const CredentialType = Object.freeze({
  Password: "password",
  Username: "username",
  Email: "email",
  SshKey: "ssh-key",
} as const);

// derive the enum-like type from the raw data
export type CredentialType = (typeof CredentialType)[keyof typeof CredentialType];
Enum-likes are structural types!

Unlike string-typed enums, enum-likes do not reify a type for each member. This means that you can use their string value or their enum member interchangeably.

let value: CredentialType = CredentialType.Username;

// this is typesafe!
value = "email";

However, the string-typed values are not always identified as enum members. Thus, when the const object is in scope, prefer it to the literal value.

Utilities​

The following utilities can be used to maintain type safety at runtime.

import { CipherType } from "./cipher-type";

const namesByCipherType = new Map<CipherType, keyof CipherType>(
  Array.fromEntries(Object.entries(CipherType), ([k, v]) => [v, k]),
);

export function isCipherType(value: number): value is CipherType {
  return namesByCipherType.has(value);
}

export function asCipherType(value: number): CipherType | undefined {
  return isCipherType(value) ? value : undefined;
}

export function nameOfCipherType(value: CipherType): keyof CipherType | undefined {
  return namesByCipherType.get(value);
}