Allow functions to have `new symbol` as the return type

Search Terms

• unique symbol
• new unique symbol
• new symbol

Suggestion

I request that unique symbol be allowed as the return type of a function declaration.

Alternatively, it might be better to use new symbol to disambiguate https://github.com/microsoft/TypeScript/issues/40106#issuecomment-677741032.

Use Cases

Currently, it’s impossible to create an alias or a wrapper function for the global Symbol constructor and use that to construct unique symbols:

// ./es-globals/fundamentals.js
export const ESSymbol = Symbol;
export const { for: ESSymbol_for } = ESSymbol;

// ./es-globals/fundamentals.d.ts
export import ESSymbol = globalThis.Symbol;
export declare const ESSymbol_for: typeof ESSymbol.for;

// ./symbols.js
import { ESSymbol, ESSymbol_for } from "./es-globals/fundamentals.js";

// should be `unique symbol`, but is instead `symbol`:
export const customSymbol = ESSymbol("custom");

// should be `unique symbol` or `global symbol "nodejs.util.inspect.custom"`,
// but is instead `symbol`:
export const nodejs_util_inspect_custom = ESSymbol_for("nodejs.util.inspect.custom");

// should be `unique symbol` or `global symbol "nodejs.util.promisify.custom"`,
// but is instead `symbol`:
export const nodejs_util_promisify_custom = ESSymbol_for("nodejs.util.promisify.custom");

Examples

This would allow defining SymbolConstructor as:

declare interface SymbolConstructor {
	/**
	 * Returns a new unique Symbol value.
	 * @param description Description of the new symbol value.
	 */
	(description?: string | number): new symbol;

	/**
	 * Returns a Symbol object from the global symbol registry matching the given key if found.
	 * Otherwise, returns a new symbol with this key.
	 * @param key key to search for.
	 */
	for<T extends string>(key: T): global symbol T;
	// or, until GH-35909 is implemented:
	for(key: string): new symbol;
}

Checklist

My suggestion meets these guidelines:

• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

See also

• https://github.com/microsoft/TypeScript/issues/36468
• https://github.com/microsoft/TypeScript/issues/35909 (global symbol "<name>" proposal)
• https://github.com/microsoft/TypeScript/pull/24738
• https://github.com/microsoft/TypeScript/issues/24506
• https://github.com/microsoft/TypeScript/issues/40106
• https://github.com/microsoft/TypeScript/pull/42543
• https://github.com/microsoft/TypeScript/issues/53282

まず、提案されている戻り値の型 unique symbol と new symbol、SymbolConstructor の例、および See also に記載されている関連する issue と pull request を確認します。意図された型のセマンティクスと影響を受けるコンパイラーの動作を特定します。完了条件には、JavaScript の出力を変更せずに、要求されたコンストラクターとレジストリ呼び出しの型付けを含める必要があります。