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Summary: `packages/metro-runtime/src/polyfills/.babelrc.js` has had no effect since 2019, and would throw if it were ever loaded. It was added (as `packages/metro/src/lib/polyfills/.babelrc`) in 2018, when the separate `build-es5` output was dropped from `scripts/build.js`, so that the published `require` polyfill was still transpiled down to ES5 via `preset-env`. After the Babel 7 upgrade it stopped applying, because Babel 7 only loads `.babelrc` files from the root package by default - that was fixed by converting it to `.babelrc.js` and adding `babelrcRoots: ['.', 'packages/*']` to the root `babel.config.js`. A few months later, the move from `babel-jest` to a custom transformer replaced `babelrcRoots` with `babelrc: false`, and it's been dead ever since. When `preset-env` was removed in 2020 it was swapped for the `@babel/plugin-proposal-nullish-coalescing-operator` and `@babel/plugin-proposal-optional-chaining` plugins, neither of which is installed any more, so `require.resolve` would fail if Babel ever did pick it up. It isn't exported or published - `npm pack metro-runtime@0.87.0 --dry-run` doesn't include it - and nothing else references it. `require.js` gets the same build as the rest of Metro, and is transformed again by the app's own Babel config when bundled. Changelog: Internal Test plan: ``` yarn jest packages/metro-runtime node scripts/build.js ``` Tests pass, and the built `packages/metro-runtime/build/polyfills/require.js` is byte-identical to a build with the file present.
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packages/metro-runtime/src/polyfills/.babelrc.jshas had no effect since 2019, and would throw if it were ever loaded, since it lists plugins we no longer have installed. It's an artifact of Metro's previous reliance onbabel-jest, before Babel 7.It isn't exported or published -
npm pack metro-runtime --dry-rundoesn't include it - and nothing else references it.require.jsalongside it is transformed the same way as any other Flow source in Metro.(Archaeology: It was added in d02b939 and became a no-op in dd0b429 . The idea was that we'd npm-publish a version of
require.jssuitable for running on device without further transformation, but we now treat it the same as any other runtime code.)Changelog: Internal
Test plan:
CI