Variable named `exports` is not type-checked

[guilhermesimoes]

Bug Report

🕗 Version & Regression Information

This is the behavior in every version I tried: v3.3.3 and v4.9.4

⏯ Playground Link

Playground link with relevant code

💻 Code

let a: number;
a = ''; // Type 'string' is not assignable to type 'number'.

let exports: number;
exports = ''; // Everything is fine

🙁 Actual behavior

Variable named exports is not type-checked.

🙂 Expected behavior

Either the variable exports should be type-checked or the variable should not be allowed to be named exports (as far as I could tell exports is not a reserved word).

[Tobias-Scholz]

It seems like this behaviour was introduced back in #23319 to support assignment to exports. If the identifier's text is just "exports", type checking is skipped.

checker.ts line 35924

if (checkReferenceExpression(left,
  Diagnostics.The_left_hand_side_of_an_assignment_expression_must_be_a_variable_or_a_property_access,
  Diagnostics.The_left_hand_side_of_an_assignment_expression_may_not_be_an_optional_property_access)
  && (!isIdentifier(left) || unescapeLeadingUnderscores(left.escapedText) !== "exports")) {
  /* ... checkTypeAssignableToAndOptionallyElaborate ... */
}        

Now I am not sure if this check !isIdentifier(left) || unescapeLeadingUnderscores(left.escapedText) !== "exports") is needed anymore. I removed it in this PR and the tests seem to pass.

[guilhermesimoes]

Fyi of how this came about. The implementation of PeacockTV and SkyShowtime uses the same codebase for multiple platforms. We have a folder structure like this:

a-component
├── index.ts
├── index.androidtv.ts
├── index.xboxone.ts
├── etc
└── index.universal.ts

Builds for a specific platform resolve to that platform file instead of the default index.ts through the usage of Webpack's resolve extensions.

This has one drawback which is that if an index.androidtv.ts export is different from an index.ts export then that's not type-checked. tsc's module resolution is different from Webpack's (since we configured it that way).

However, we have a "universal" build that imports code from all platforms and that at runtime can switch between them. This is not intended for production but is useful for testing on multiple platforms. Here's an example:

import { Platform } from './shared/platform';
import * as defaultExports from './index';
import * as androidtvExports from './index.androidtv';
import * as xboxoneExports from './index.xboxone';

let exports: typeof defaultExports;

switch (self.platform) {
  case Platform.AndroidTv:
    exports = androidtvExports;
    break;

  case Platform.XboxOne:
    exports = xboxoneExports;
    break;

  // etc

  default:
    exports = defaultExports;
}

export const initialiseDevice = exports.initialiseDevice;
export const getDeviceCapabilities = exports.getDeviceCapabilities;

So I was looking at this code and started to wonder "wait, why the hell don't we have type-safety on the exports assignments inside the switch?". And that's when I started to play with that variable and figured it wasn't checked at all.

In the mean time, we'll probably rename the variables exports on our side to fix any eventual incompatibilities we find so that we are ready for a TS version with this fix.

But thank you for looking into this so quickly! 😀