feat(polyfills): introduce @rnx-kit/polyfills

.changeset/angry-impalas-sneeze.md

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+---
+---

incubator/polyfills/README.md

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+# @rnx-kit/polyfills
+
+[![Build](https://github.com/microsoft/rnx-kit/actions/workflows/build.yml/badge.svg)](https://github.com/microsoft/rnx-kit/actions/workflows/build.yml)
+[![npm version](https://img.shields.io/npm/v/@rnx-kit/polyfills)](https://www.npmjs.com/package/@rnx-kit/polyfills)
+
+🚧🚧🚧🚧🚧🚧🚧🚧🚧🚧🚧
+
+### THIS TOOL IS EXPERIMENTAL — USE WITH CAUTION
+
+🚧🚧🚧🚧🚧🚧🚧🚧🚧🚧🚧
+
+This is a polyfills "autolinker" for Metro. It works like native module
+autolinking, but gathers polyfills from dependencies instead.
+
+> **Note**
+>
+> This package is temporary. Ideally, this should be upstreamed to
+> `@react-native-community/cli`.
+
+## Motivation
+
+Please read the
+[Modularity](https://github.com/microsoft/rnx-kit/blob/tido/react-native-standard-api/text/0002-react-native-webapis.md#modularity)
+section of the
+[React Native WebAPIs RFC](https://github.com/microsoft/rnx-kit/pull/2504) for
+its raison d'être.
+
+## Installation
+
+```sh
+yarn add @rnx-kit/polyfills --dev
+```
+
+or if you're using npm
+
+```sh
+npm add --save-dev @rnx-kit/polyfills
+```
+
+## Usage
+
+```diff
+ const { makeMetroConfig } = require("@rnx-kit/metro-config");
+ const { getPreludeModules } = require("@rnx-kit/polyfills");
+
+ module.exports = makeMetroConfig({
++  serializer: {
++    getModulesRunBeforeMainModule: getPreludeModules,
++  },
+ });
+```

incubator/polyfills/package.json

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+{
+  "private": true,
+  "name": "@rnx-kit/polyfills",
+  "version": "0.0.1",
+  "description": "EXPERIMENTAL - USE WITH CAUTION - New package called polyfills",
+  "homepage": "https://github.com/microsoft/rnx-kit/tree/main/incubator/polyfills#readme",
+  "license": "MIT",
+  "author": {
+    "name": "Microsoft Open Source",
+    "email": "microsoftopensource@users.noreply.github.com"
+  },
+  "files": [
+    "lib/*"
+  ],
+  "main": "lib/index.js",
+  "types": "lib/index.d.ts",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/microsoft/rnx-kit",
+    "directory": "incubator/polyfills"
+  },
+  "engines": {
+    "node": ">=14.15"
+  },
+  "scripts": {
+    "build": "rnx-kit-scripts build",
+    "format": "rnx-kit-scripts format",
+    "lint": "rnx-kit-scripts lint"
+  },
+  "dependencies": {
+    "@rnx-kit/console": "^1.0.0",
+    "@rnx-kit/tools-node": "^2.0.0"
+  },
+  "peerDependencies": {
+    "@react-native/js-polyfills": "*"
+  },
+  "peerDependenciesMeta": {
+    "@react-native/js-polyfills": {
+      "optional": true
+    }
+  },
+  "devDependencies": {
+    "@rnx-kit/scripts": "*",
+    "eslint": "^8.0.0",
+    "jest": "^29.2.1",
+    "metro-config": "^0.73.7",
+    "prettier": "^3.0.0",
+    "typescript": "^5.0.0"
+  },
+  "eslintConfig": {
+    "extends": "@rnx-kit/eslint-config"
+  },
+  "jest": {
+    "preset": "@rnx-kit/scripts"
+  },
+  "rnx-kit": {
+    "alignDeps": {
+      "presets": [
+        "microsoft/react-native",
+        "@rnx-kit/scripts/align-deps-preset.js"
+      ],
+      "requirements": [
+        "react-native@0.71"
+      ],
+      "capabilities": [
+        "metro-config"
+      ]
+    }
+  },
+  "experimental": true
+}

incubator/polyfills/src/dependency.ts

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+import { error } from "@rnx-kit/console";
+import { readPackage } from "@rnx-kit/tools-node";
+import * as path from "path";
+import type { Context } from "./types";
+
+function getDependencies({ projectRoot }: Context): string[] {
+  const manifest = readPackage(projectRoot);
+
+  const dependencies = new Set<string>();
+  for (const section of ["dependencies", "devDependencies"] as const) {
+    const names = manifest[section];
+    if (names) {
+      Object.keys(names).forEach((name) => dependencies.add(name));
+    }
+  }
+
+  return Array.from(dependencies);
+}
+
+function isValidPath(p: string): boolean {
+  return (
+    Boolean(p) &&
+    !p.startsWith("..") &&
+    !p.startsWith("/") &&
+    !/^[A-Za-z]:/.test(p)
+  );
+}
+
+export function getDependencyPolyfills(context: Context): string[] {
+  const polyfills: string[] = [];
+
+  const options = { paths: [context.projectRoot] };
+  const dependencies = getDependencies(context);
+
+  for (const name of dependencies) {
+    try {
+      const config = require.resolve(`${name}/react-native.config.js`, options);
+      const polyfill = require(config).dependency?.api?.polyfill;
+      if (typeof polyfill === "string") {
+        if (!isValidPath(polyfill)) {
+          error(`${name}: invalid polyfill path: ${polyfill}`);
+          continue;
+        }
+
+        polyfills.push(path.resolve(path.dirname(config), polyfill));
+      }
+    } catch (_) {
+      // ignore
+    }
+  }
+
+  return polyfills;
+}

incubator/polyfills/src/index.ts

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+import { getDependencyPolyfills } from "./dependency";
+import type { Context, GetPreludeModules } from "./types";
+
+/**
+ * Ideally, we'd need something between `serializer.getPolyfills` and
+ * `serializer.getModulesRunBeforeMainModule`. The former does not have access
+ * to `require`, while the latter requires that the listed modules are
+ * explicitly used in the bundle itself (see
+ * https://github.com/facebook/metro/issues/850). For now, we will use this fact
+ * to simply list all prelude modules.
+ */
+function defaultModules({ projectRoot }: Context): string[] {
+  const platforms = [
+    "react-native",
+    "react-native-macos",
+    "react-native-windows",
+  ];
+  const options = { paths: [projectRoot] };
+
+  const modules = [];
+  for (const platform of platforms) {
+    const core = `${platform}/Libraries/Core/InitializeCore`;
+    try {
+      modules.push(require.resolve(core, options));
+    } catch (_) {
+      // ignore
+    }
+  }
+
+  return modules;
+}
+
+export const getPreludeModules: GetPreludeModules = () => {
+  const context = { projectRoot: process.cwd() };
+  const modules = defaultModules(context);
+  const dependencyPolyfills = getDependencyPolyfills(context);
+  return modules.concat(dependencyPolyfills);
+};
+
+export default getPreludeModules;

incubator/polyfills/src/types.ts

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+import type { ConfigT } from "metro-config";
+
+export type Context = {
+  projectRoot: string;
+};
+
+export type GetPreludeModules =
+  ConfigT["serializer"]["getModulesRunBeforeMainModule"];

incubator/polyfills/tsconfig.json

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+{
+  "extends": "@rnx-kit/scripts/tsconfig-shared.json",
+  "include": ["src"]
+}

yarn.lock

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   languageName: unknown
   linkType: soft
 
+"@rnx-kit/polyfills@workspace:incubator/polyfills":
+  version: 0.0.0-use.local
+  resolution: "@rnx-kit/polyfills@workspace:incubator/polyfills"
+  dependencies:
+    "@rnx-kit/console": ^1.0.0
+    "@rnx-kit/scripts": "*"
+    "@rnx-kit/tools-node": ^2.0.0
+    eslint: ^8.0.0
+    jest: ^29.2.1
+    metro-config: ^0.73.7
+    prettier: ^3.0.0
+    typescript: ^5.0.0
+  peerDependencies:
+    "@react-native/js-polyfills": "*"
+  peerDependenciesMeta:
+    "@react-native/js-polyfills":
+      optional: true
+  languageName: unknown
+  linkType: soft
+
 "@rnx-kit/react-native-auth@*, @rnx-kit/react-native-auth@workspace:packages/react-native-auth":
   version: 0.0.0-use.local
   resolution: "@rnx-kit/react-native-auth@workspace:packages/react-native-auth"