experimentalImportSupport: Place export-all assignments before explicit exports, fix precedence of export default > export * from

[robhogan]

Summary:
Fixes a bug in experimentalImportSupport where export * from 'foo' would incorrectly overwrite an export default, if 'foo' also has a default export.

Example

Input source

// foo.js  (the star source)
export default 'star default';
export const overridden = 'star named';
export const sourceOnly = 'source only';

// entry.js
export * from 'foo';
export const overridden = 'explicit named';
export default 'explicit default';

Per the ES spec, importing from entry.js should give overridden === 'explicit named', default === 'explicit default', and sourceOnly === 'source only'. Explicit exports take precedence over names supplied via export *, regardless of the position of export declarations.

Before

'use strict';

Object.defineProperty(exports, '__esModule', {value: true});

var _overridden = 'explicit named';
var _default = 'explicit default';

exports.overridden = _overridden;    // 'explicit named'
exports.default = _default;          // 'explicit default'

var _foo = require('foo');
for (var _key in _foo) {             // star copy runs LAST...
  exports[_key] = _foo[_key];        // ...clobbers the explicit exports
}

Runtime result (wrong):

exports.overridden // 'star named'     ❌ should be 'explicit named'
exports.default    // 'star default'   ❌ should be 'explicit default'
exports.sourceOnly // 'source only'    ✅

After

'use strict';

Object.defineProperty(exports, '__esModule', {value: true});

var _overridden = 'explicit named';
var _default = 'explicit default';

var _foo = require('foo');
for (var _key in _foo) {             // star copy runs FIRST
  exports[_key] = _foo[_key];
}

exports.overridden = _overridden;    // explicit exports now win
exports.default = _default;

Runtime result (spec-correct):

exports.overridden // 'explicit named'    ✅
exports.default    // 'explicit default'  ✅
exports.sourceOnly // 'source only'       ✅

Spec

ECMA-262 Source Text Module Record ResolveExport checks local ExportEntries and indirect ExportEntries before considering StarExportEntries, so explicit exports take precedence over names supplied through export *.

Provenance

This is part of the change made in Expo's fork in #38111 (though that's mainly concerned with introducing live bindings), so this is "in the wild" already for Expo users.

Implementation

This is just a straightforward ordering swap of the emitted assignments.

Changelog

 - **[Experimental]**: `experimentalImportSupport` - fix precedence of `export default` over `export * from`.

Reviewed By: huntie

Differential Revision: D111013892

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D111013892.

[meta-codesync]

This pull request has been merged in e6b154f.