Typescript: When not to use enums

I wrote this post as a reminder to myself that using const enums in a library is a really bad idea.

So in short. If you are writing a library and you export a const enum then some developers will not be able to compile their applications if they import your library. Especially when using Babel. Here we look at why.

Enums: handle with care

When you declare an enum, Typescript will generate some code for it. This is fine if you are writing an application code and you do not care what is generated. But when you are writing a library you sometimes do not need the features provided by this style of declarations - you just want to use enums instead of constants.

This has a solution - you can use const enum instead of an enum. Typescript does not generate code for const enum declaration. This is great - it is just like using constant - but there is a problem.

Isolated modules

When you are writing a library you do not have control over your code and how is used or compiled. Nor what compiler is used and how. Especially Babel and other transpilers operate over one file at a time. That means that the type information are stripped and a code transformation that depends on understanding the full type system cannot be applied. Simply const enum imported to another file is gone.

To prevent this error in libraries you should use --isolatedModules option in your tsconfig.json file.

Further reading

• You can read a more detailed explanation in typescript documentation or in the article Typescript: Don't export const enums.

Prefer union types over enums

enum and const enum are features from "the old days" of the Typescript where the Javascript landscape was a lot different. In addition, enums generate a lot of code you probably do not want. And also numeric enums are not type-safe. And instead of key-value-based enums you can use an object which will serve the same.

const Direction = {
  Up: 0,
  Down: 1,
  Left: 2,
  Right: 3,
} as const;

// Get to the const values of any object
type Values<T> = T[keyof T];

// Values<typeof Direction> yields 0 | 1 | 2 | 3
declare function move(direction: Values<typeof Direction>): void;

move(30);
//   ^ 💥 This breaks!

move(0);
//   ^ 👍 This works!

move(Direction.Left);
//   ^ 👍 This also works!

// And now for the Status enum

const Status = {
  Admin: 'Admin',
  User: 'User',
  Moderator: 'Moderator',
} as const;

// Values<typeof Status> yields "Admin" | "User" | "Moderator"
declare function closeThread(threadId: number, status: Values<typeof Status>): void;

closeThread(10, 'Admin'); // All good!
closeThread(10, Status.User); // enum style

A better approach is to use union types. A simple union type gives you something that works similary and is much more aligned with Typescript.

type Status = 'Admin' | 'User' | 'Moderator';

declare function closeThread(threadId: number, status: Status): void;

closeThread(10, 'Admin');
// All good :-)

Further reading

• Detailed coverage of this topic is in the acticle Tidy Typescript: Avoid Enum.

Conclusion

• use enum or const enum in application code
• set --isolatedModules in tsconfig.json to true in libraries
• prefer value object if you want a key-value based enum
• prefer union types generally over enums

References

• Ambient const enums are not allowed when the '--isolatedModules' flag is provided - Why enums are not allowed with --isolatedModules
• Tidy Typescript: Avoid Enums - More about how to use union types instead of enums
• Don't export const enums - More about why const enum does not work in libraries
• tsconfig reference: isolated modules
• Solved problem with enums in LMC's library - How we solve the problem with enum in our library