Dual packaging, without breaking existing consumers (#628)

Co-authored-by: Yasumasa Ashida <ys.ashida@gmail.com>

tsdown.config.ts

@@ -0,0 +1,107 @@
+import { defineConfig } from 'tsdown';
+import babel from '@rolldown/plugin-babel';
+import { minify } from 'terser';
+
+const banner = `/*!
+ * Bowser - a browser detector
+ * https://github.com/lancedikson/bowser
+ * MIT License | (c) Dustin Diaz 2012-2015
+ * MIT License | (c) Denis Demchenko 2015-2026
+ */`;
+
+/**
+ * The browser targets the UMD bundles are transpiled down to. Unchanged from
+ * the webpack build these bundles replaced — the published `es5.js` is relied
+ * on by script-tag and CDN consumers on very old browsers.
+ */
+const legacyTargets = {
+  ie: '8',
+  browsers: '>2%',
+};
+
+/**
+ * `useBuiltIns: false` for `es5.js` (syntax transpilation only) and `'entry'`
+ * for `bundled.js`, which expands the `core-js/stable` import in its entry.
+ */
+const legacyBabel = (useBuiltIns: false | 'entry') => babel({
+  presets: [['@babel/preset-env', {
+    // Let rolldown emit the UMD wrapper; babel only lowers syntax here.
+    modules: false,
+    loose: true,
+    useBuiltIns,
+    ...(useBuiltIns ? { corejs: '3' } : {}),
+    targets: legacyTargets,
+  }]],
+});
+
+/**
+ * Minifies the UMD chunks with terser instead of rolldown's built-in (oxc)
+ * minifier.
+ *
+ * This is not a preference. oxc's minifier prints every string literal as a
+ * template literal and refuses any `compress.target` below `es2015`, so it
+ * cannot emit ES5 — it would silently undo babel's lowering and break the very
+ * old browsers `es5.js` exists to serve. terser (`ecma: 5`) is also what the
+ * webpack 4 build this replaced used, via terser-webpack-plugin.
+ */
+const terser = () => ({
+  name: 'bowser:terser',
+  async renderChunk(code: string, chunk: { fileName: string }) {
+    const result = await minify(code, {
+      ecma: 5,
+      // IE 8: reserved words as property names must stay quoted.
+      ie8: true,
+      safari10: true,
+      format: {
+        // Keep the `/*!` banner.
+        comments: /^!/,
+      },
+    });
+    if (typeof result.code !== 'string') {
+      throw new Error(`terser produced no output for ${chunk.fileName}`);
+    }
+    return { code: result.code };
+  },
+});
+
+const umd = (name: string, entry: string, useBuiltIns: false | 'entry') => ({
+  entry: { [name]: entry },
+  format: ['umd' as const],
+  // Lowercase, matching the global the webpack build published. Renaming this
+  // to `Bowser` would break every `<script src=".../bowser/es5.js">` consumer.
+  globalName: 'bowser',
+  outputOptions: {
+    // `module.exports = Bowser` rather than `{ default: Bowser }`.
+    exports: 'default' as const,
+    // Default would be `<name>.umd.js`; these files are published paths.
+    entryFileNames: '[name].js',
+  },
+  outDir: '.',
+  platform: 'browser' as const,
+  plugins: [legacyBabel(useBuiltIns), terser()],
+  // webpack ran in `mode: 'production'`; minification happens in `terser()`
+  // above, so rolldown's own minifier stays off. See its comment for why.
+  minify: false,
+  banner,
+  dts: false,
+  // outDir is the repo root — never let tsdown clean it.
+  clean: false,
+});
+
+export default defineConfig([
+  umd('es5', 'build/entries/es5.js', false),
+  umd('bundled', 'build/entries/bundled.js', 'entry'),
+  {
+    // Modern ESM build. Reached via the `import` condition of the exports map.
+    // Deliberately not run through babel: `module` still points at the raw
+    // `src/bowser.js`, so this must not be *more* conservative than that.
+    entry: { bowser: 'src/bowser.js' },
+    format: ['esm'],
+    outDir: '.',
+    outExtensions: () => ({ js: '.mjs' }),
+    platform: 'browser',
+    banner,
+    dts: false,
+    clean: false,
+  },
+]);