[vzaidman]

Summary:

Rewrites the docs for React Native developers who know Metro from the product side (metro.config.js, npx react-native start, Fast Refresh) but not its internals, and who often land on a single section from search. Builds on #2077 and #2084, which fixed inaccuracies and documented missing options.

• Concepts as a glossary: Concepts.md defines the bundling stages and shared terms (module, entry point, dependency graph, platform, project root and watch folders, file watching, Haste) once, and each stage says how to customize it. Other pages link to these definitions instead of re-explaining them.
• Standalone pages and sections: each page opens with what it covers and when you'd need it; each config section says what it controls.
• Vanilla Metro first: examples run Metro directly (npx metro serve, npx metro build, plain config exports), and note where a framework wrapper (e.g. React Native's or Expo's getDefaultConfig) changes a default instead of assuming it. The JavaScript API is framed as a framework/tooling integration point, not the usual way to run Metro.
• Less duplication: API.md is the single reference for the JavaScript API. GettingStarted.md is a guided introduction with short examples (the CLI, createConnectMiddleware, metro.config.js) that links to it.
• Bundling page: "Bundle Formats" becomes "Bundling": how Metro builds a bundle (stages, module IDs, serialization order), modules and scripts (with a polyfill example), then the formats. The RAM bundle section explains when RAM bundles help and why React Native doesn't use them with Hermes, with an example and a compact description of both formats.
• Concrete examples where a statement isn't obvious, e.g. the URLs Metro's middleware serves, what a wrapped module looks like in a bundle, how a specifier resolves to a file, mergeConfig results, and the files runBuild writes.
• Recent behavior: inlineRequires inlining destructured requires, fileMapCacheDirectory being created if missing, and what getPackageForModule returns for symlinked and missing paths.
• New "File watching" section in Concepts.md, linked from every mention of watching.
• Tightened prose throughout, consistent "Type:" / "Defaults to" phrasing, and removed trailing whitespace.

Page URLs are unchanged, and headings keep their text where possible, so existing links keep working. The exceptions: Bundling.md's sections are reorganized (the format headings keep their text), and Concepts.md's "Modules" heading is now "Module", under "Terminology".

Changelog: [Internal]

Differential Revision: D125674124

[meta-codesync]

@vzaidman has exported this pull request. If you are a Meta employee, you can view the originating Diff in D125674124.

[robhogan]

React Native context: examples use mergeConfig(getDefaultConfig(__dirname), ...) from react-native/metro-config, and options note where React Native's defaults differ from Metro's.

I'm not sure about this as a goal - those are really community CLI specifics, at a time when we're actively recommending a framework. I'd be inclined to pick either vanilla Metro or represent Expo alongside RN CLI.

[huntie]

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