Allow merging enums with undefined

🔍 Search Terms

enum merge undefined

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

It can sometimes be useful to augment the types for a module to indicate that an exported enum is optional. There's no way I can find in TypeScript to do that right now. "Enum declarations can only merge with namespace or other enum declarations."

📃 Motivating Example

For example API consumers may need to support versions of a package that don't include a recently added enum.

How about allowing a ? before the { for an enum?

declare module 'typescript' {
  enum JSDocParsingMode? {}
}

💻 Use Cases

In typescript-eslint we want to always treat ts.JSDocParsingMode as optional. It was added in a more recent version of TypeScript than our backwards compatibility extends. But there's no way to mark it as | undefined.

Even if we do remember to always ts.JSDocParsingMode?.ParseAll and so on, doing so violates https://typescript-eslint.io/rules/no-unnecessary-condition.

Es werden keine Dateien, Tests oder spezifischen Einstiegspunkte genannt. Untersuche zunächst, wie Enum-Deklarationen zusammengeführt werden und wie die zugrunde liegende Modulaugmentierung typgeprüft wird. Als erledigt gilt, wenn es eine unterstützte Möglichkeit gibt, das exportierte Enum als optional zu kennzeichnen und dabei die genannten Kompatibilitäts- und Laufzeitbeschränkungen einzuhalten.