Suggestion: Allow ES6 export syntax in namespaces

TypeScript Version: typescript@4.0.0-dev.20200801

Search Terms: namespace, import, export, re-export, module

Code

import { Point } from "./Point";

export type Line = {
  readonly p1: Point;
  readonly p2: Point;
  readonly points?: readonly [Point, ...(readonly Point[])];
};

export namespace Line {
  export { Point } from "~/Point"; // TS says "Export declarations are not permitted in a namespace"
  // `export { Point };` <-- Should also work.
}

type LinePoint = Line.Point;
// ^ Somehow this actually works, i.e. `LinePoint` equals `Point`

Expected behavior: Exporting from a namespace should mirror ES6 module exports, i.e. export { X }, export { X } from "...", and export * as NS from "..."

The lack of syntax for re-exports makes it very hard to use namespace/type/function merging abilities to represent nice APIs. Here’s an example of what a module could look like if this was allowed…

// User.tsx

import { Admin } from "./Admin";
import { Member } from "./Member";

export type User = Admin | Member;
export type Props = { readonly user: User; };

export function User({ user }: Props) {
  return Admin.isAdmin(user) ? <Admin admin={user} /> : <Member member={user} />;
}

export namespace User {
  export { Admin, Member }; // Nice.
  export const isAuthenticated = (user: User): boolean => true;
}

// Home.tsx

import { User } from "./User";

export function Home({ user }: User.Props) {
  return <User.Admin admin={useAdmin()} />; // An example of what this enables.
}

Actual behavior: TypeScript says “Export declarations are not permitted in a namespace,” even though it actually respects the export declaration outside of the namespace.

Playground Link: https://www.typescriptlang.org/play/#code/HYQwtgpgzgDiDGEAEBBJBvAUEnSIA8YB7AJwBcl4jgoKQkBeJAIhGaRCkutoG5MAvpkyhIsBMgBCGbLgLFyGVEgH8hmKjQr5GSSQDoU+kLyA

Related Issues: https://github.com/microsoft/TypeScript/issues/20273 This issue showcases some of the syntax gymnastics needed to get around this issue.

https://github.com/microsoft/TypeScript/issues/4529 https://github.com/microsoft/TypeScript/issues/4529#issuecomment-140932811 More examples of verbose workarounds.

https://github.com/microsoft/TypeScript/issues/38041#issuecomment-616807133 @rbuckton Provides a nice example of what this could look like if enabled.

https://github.com/microsoft/TypeScript/issues/38041#issuecomment-662142857 My comment with another motivating example after the issue was closed.

[cruhl]

Here’s more “in the wild” code I found showing the syntax noise needed to get around this…

// This file is "./Claim/index.ts"

import * as Details_ from "./Details";
import * as Dashboard_ from "./Dashboard";
import * as Create_ from "./Create";
import * as Admin_ from "./Admin";

import { API } from "../API";

export type Claim = API.GetClaimQuery["getClaim"];
export namespace Claim {
  export const Create = Create_;
  export const Details = Details_;
  export const Dashboard = Dashboard_;
  export const Admin = Admin_;
}

Here’s what it could look like…

import { API } from "../API";

export type Claim = API.GetClaimQuery["getClaim"];
export namespace Claim {
  export * as Create from "./Create";
  export * as Details from "./Details";
  export * as Dashboard from "./Dashboard";
  export * as Admin from "./Admin";
}

Or even…

import * as Details from "./Details";
import * as Dashboard from "./Dashboard";
import * as Create from "./Create";
import * as Admin from "./Admin";

import { API } from "../API";

export type Claim = API.GetClaimQuery["getClaim"];
export namespace Claim {
  export { Details, Dashboard, Create, Admin };
}

[cruhl]

@andrewbranch I apologize if tagging you directly isn’t the right move, let me know if that’s the case, but I was wondering if you could provide some insight since you recently closed another related issue.

This is still something I believe would be a useful change and I’m eager/willing to champion/help in whatever way I can. Any pointers for moving the issue forward? Thanks in advance!