misc: prepare core/lib/legacy-javascript to be published to npm (#16416)

Author: connorjclark

build/build-legacy-javascript.js

@@ -0,0 +1,32 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import fs from 'fs';
+
+import esbuild from 'esbuild';
+import esMain from 'es-main';
+
+import {LH_ROOT} from '../shared/root.js';
+
+async function buildPackage() {
+  await esbuild.build({
+    entryPoints: ['core/lib/legacy-javascript/legacy-javascript.js'],
+    outfile: 'dist/legacy-javascript/legacy-javascript.js',
+    format: 'esm',
+    bundle: true,
+  });
+
+  fs.copyFileSync(`${LH_ROOT}/core/lib/legacy-javascript/package.json`,
+    'dist/legacy-javascript/package.json');
+}
+
+async function main() {
+  await buildPackage();
+}
+
+if (esMain(import.meta)) {
+  await main();
+}

core/lib/legacy-javascript/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "legacy-javascript",
+  "type": "module",
+  "version": "0.0.1",
+  "description": "Identify legacy polyfills and code transforms",
+  "main": "./legacy-javascript.js",
+  "repository": "GoogleChrome/lighthouse",
+  "author": "Google LLC",
+  "license": "Apache-2.0",
+  "bugs": {
+    "url": "https://github.com/GoogleChrome/lighthouse/issues"
+  },
+  "homepage": "https://github.com/GoogleChrome/lighthouse#readme"
+}

package.json

@@ -16,14 +16,15 @@
     "prepack": "yarn build-report --standalone --flow --esm && yarn build-types",
     "postpack": "yarn clean-types",
     "build-all": "npm-run-posix-or-windows build-all:task",
-    "build-all:task": "yarn build-report && yarn build-cdt-strings && yarn build-devtools && concurrently 'yarn build-extension' 'yarn build-lr' 'yarn build-viewer' 'yarn build-treemap' 'yarn build-smokehouse-bundle' && yarn build-pack",
-    "build-all:task:windows": "yarn build-report && yarn build-cdt-strings && yarn build-extension && yarn build-devtools && yarn build-lr && yarn build-viewer && yarn build-treemap && yarn build-smokehouse-bundle",
+    "build-all:task": "yarn build-report && yarn build-cdt-strings && yarn build-devtools && concurrently 'yarn build-extension' 'yarn build-lr' 'yarn build-viewer' 'yarn build-treemap' 'yarn build-smokehouse-bundle' 'yarn build-legacy-javascript' && yarn build-pack",
+    "build-all:task:windows": "yarn build-report && yarn build-cdt-strings && yarn build-extension && yarn build-devtools && yarn build-lr && yarn build-viewer && yarn build-treemap && yarn build-smokehouse-bundle && yarn build-legacy-javascript",
     "build-cdt-lib": "node ./build/build-cdt-lib.js",
     "build-cdt-strings": "node ./build/build-cdt-strings.js",
     "build-extension": "yarn build-extension-chrome && yarn build-extension-firefox",
     "build-extension-chrome": "node ./build/build-extension.js chrome",
     "build-extension-firefox": "node ./build/build-extension.js firefox",
     "build-devtools": "yarn reset-link && node ./build/build-bundle.js clients/devtools/devtools-entry.js dist/lighthouse-dt-bundle.js && node ./build/build-dt-report-resources.js",
+    "build-legacy-javascript": "node ./build/build-legacy-javascript.js",
     "build-smokehouse-bundle": "node ./build/build-smokehouse-bundle.js",
     "build-lr": "yarn reset-link && node --max-old-space-size=4096 ./build/build-lightrider-bundles.js",
     "build-pack": "bash build/build-pack.sh",