Anonymous Symbol Properties

Issue にある現在の型宣言と提案された型宣言から始め、TypeScript が計算されたプロパティキーと unique symbol のアイデンティティをどのように扱うかを追跡します。inline の unique symbol プロパティキーを受け入れ、各宣言に新しいアイデンティティを付与し、実行時の JavaScript 出力を維持できれば完了です。この issue では実装ファイルやテストは指定されていません。

🔍 Search Terms

"anonymous symbol properties"
"anonymous unique symbol"
"inline unique symbol"
"unique symbol property key"
"computed property unique symbol"
"symbol branding"

✅ 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

Allow unique symbol keys to be declared inline at the property declaration site.

📃 Motivating Example

Current code:

declare const brand: unique symbol;
type UserId = string & { readonly [brand]: "UserId" };

Proposed code:

type UserId = string & { readonly [brand: unique symbol]: "UserId" };

Each inline declaration would create a fresh symbol identity, equivalent to declaring a separate unique symbol and using it as the computed property key.

This is useful when a unique symbol property key is needed only once and does not need to be referenced independently. Common examples include phantom properties used for branded or refined types.

💻 Use Cases

1. What do you want to use this for?
   Declaring single-use unique symbol property keys inline, particularly for phantom properties used by branded or refined types.

2. What shortcomings exist with current approaches?
   The current approach requires introducing a value-level declaration solely to provide a unique identity for a type-level property key. For single-use symbol keys, this creates an otherwise unnecessary binding in the surrounding scope.

3. What workarounds are you using in the meantime?
   Declaring a separate unique symbol value and using it as a computed property key.