Hoist re-export dependencies before module body#1774
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Summary: In ESM, static imports (including `export * from`) must be evaluated before the current module's body. Metro's `import-export-plugin` currently evaluates named re-exports at their declared position within the body (by turning them into a CJS `require` at that posiiton), and `export *` after the body - both are incorrect. Move static re-export helper requires and export-star copies into the import prelude instead of leaving them at the original export declaration location or after the module body. ## `inlineRequires` Under `inlineRequires`, which runs after this plugin, evaluation of imported modules may be deferred back to where they were before this diff - contrary to the spec but by design for startup performance. ## Spec - Source Text Module Records collect imported and re-exported module specifiers as requested modules: https://tc39.es/ecma262/#sec-source-text-module-records - ECMA-262 `InnerModuleEvaluation` evaluates requested modules before executing the current module body: https://tc39.es/ecma262/#sec-innermoduleevaluation ## Expo Along with the previous diff, this is another change introduced in Expo last year in [#38111](expo/expo#38111), on by default for Expo apps. ## Changelog ```javascript - **[Experimental]**: `experimentalImportSupport`: Move `export from` imports to be evaluated at module prelude, rather than within/after module body ``` Reviewed By: huntie Differential Revision: D111013896
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This pull request has been merged in 27a5adc. |
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Summary:
In ESM, static imports (including
export * from) must be evaluated before the current module's body.Metro's
import-export-plugincurrently evaluates named re-exports at their declared position within the body (by turning them into a CJSrequireat that posiiton), andexport *after the body - both are incorrect.Move static re-export helper requires and export-star copies into the import prelude instead of leaving them at the original export declaration location or after the module body.
inlineRequiresUnder
inlineRequires, which runs after this plugin, evaluation of imported modules may be deferred back to where they were before this diff - contrary to the spec but by design for startup performance.Spec
InnerModuleEvaluationevaluates requested modules before executing the current module body: https://tc39.es/ecma262/#sec-innermoduleevaluationExpo
Along with the previous diff, this is another change introduced in Expo last year in #38111, on by default for Expo apps.
Changelog
Reviewed By: huntie
Differential Revision: D111013896