Support symbol properties on namespaces

Search Terms

• namespace symbol
• namespace symbol property
• namespace symbol properties

Suggestion

The ability to declare symbol properties on TypeScript namespaces.

Use Cases

See https://github.com/DefinitelyTyped/DefinitelyTyped/pull/42154#discussion_r376739320.

Examples

// doSomething.d.ts
import { promisify } from "util";

declare function doSomething(
	foo: any,
	onSuccessCallback: (result: string) => void,
	onErrorCallback: (reason: any) => void
);
declare namespace doSomething {
	function [promisify.custom](foo: any): Promise<string>;
}

export = doSomething;

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.

[DanielRosenwasser]

Seems like this is mostly meant for the .d.ts representations - is that the intent? Because otherwise, you can already add computed properties to functions.

function blah() {

}

blah[Symbol.iterator] = 100;

One problem that @weswigham brought up with this is that assuming you’re repurposing let/const/var to export these symbols, it would introduce a parsing ambiguity.

export namespace foo {
  export let [xyz] = 10; // currently parses as a binding pattern
}

[weswigham]

We could allow it only in ambient contexts, but that has the possibility of stepping on whatever tc39 might do to allow symbol-named module members natively.

😦