Should create unique symbols when creating properties on module.exports

[ExE-Boss]

TypeScript Version:

3.9.0-dev.20200315

Search Terms:

• module.exports unique symbol property
• module.exports unique symbol
• exports unique symbol property
• exports unique symbol

Code

// @target: ES2015
// @module: CommonJS
// @allowJs
// @checkJs
// @noEmit
// @filename: index.js

exports.customSymbol = Symbol("custom");
//                ^?

Workbench Repro

Expected behavior:

// some-module.d.ts
export const customSymbol: unique symbol;

Actual behavior:

// some-module.d.ts
export var customSymbol: symbol;

Playground Link:

https://www.typescriptlang.org/play?module=1&useJavaScript=true#code/KYDwDg9gTgLgzgOgMYFc4wgWwMoE9MBGEANgAQC8pehJAFAESrpb0CUA3AFBA

Related Issues:

• Should preserve unique symbols when re‑assigning module.exports #37403

[RyanCavanaugh]

@rbuckton @weswigham what do we think should happen here?

[weswigham]

We don't make unique symbols because we see the position as mutable, ie,

exports.customSymbol = Symbol("custom1")
// later
exports.customSymbol = Symbol("custom2")

is perfectly acceptable.

In theory, we could recognize

Object.defineProperty(module.exports, "customSymbol", {
  writable: false,
  value: Symbol("custom")
});

as making the customSymbol property immutable and thus the symbol appropriately unique, but we currently do not.

[ExE-Boss]

But TypeScript complains when you re‑assign a property on module.exports with:

TS2323: Cannot redeclare exported variable 'customSymbol'.

Also, that code looks like a bug to me, so I don’t think it should be the default.

---

Also, if you do: (🔗)

const customSymbol = Symbol("custom");
exports.customSymbol = customSymbol;

Then you get:

export var customSymbol: typeof customSymbol;
declare const customSymbol: unique symbol;
export {};

[weswigham]

Hm. I didn't realize we issued the Cannot redeclare exported variable 'customSymbol'. error - I'm surprised we do, actually, considering there's no such limitation for cjs modules. Afaik we shouldn't be.

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in this issue running against the nightly TypeScript.

---

Issue body code block by @ExE-Boss

⚠️ Assertions:

• (property) customSymbol: symbol

Historical Information

Issue body code block by @ExE-Boss

Version	Reproduction Outputs
3.8.2, 3.9.2, 4.0.2, 4.1.2, 4.2.2, Nightly	⚠️ Assertions: (property) customSymbol: symbol