feat(lib): enable minification but keep pure annotations for es output with terser (#20522)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: sapphi-red <[email protected]>

Author: Copilot

packages/vite/src/node/plugins/terser.ts

@@ -97,12 +97,6 @@ export function terserPlugin(config: ResolvedConfig): Plugin {
         return null
       }
 
-      // Do not minify ES lib output since that would remove pure annotations
-      // and break tree-shaking.
-      if (config.build.lib && outputOptions.format === 'es') {
-        return null
-      }
-
       // Lazy load worker.
       worker ||= makeWorker()
 
@@ -111,6 +105,14 @@ export function terserPlugin(config: ResolvedConfig): Plugin {
         const res = await worker.run(terserPath, code, {
           safari10: true,
           ...terserOptions,
+          format: {
+            ...terserOptions.format,
+            // For ES lib mode, preserve comments to keep pure annotations for tree-shaking
+            preserve_annotations:
+              config.build.lib && outputOptions.format === 'es'
+                ? true
+                : terserOptions.format?.preserve_annotations,
+          },
           sourceMap: !!outputOptions.sourcemap,
           module: outputOptions.format.startsWith('es'),
           toplevel: outputOptions.format === 'cjs',

playground/lib/__tests__/lib.spec.ts

@@ -82,6 +82,21 @@ describe.runIf(isBuild)('build', () => {
     expect(umd).toMatch('process.env.NODE_ENV')
   })
 
+  test('debugger statements are removed by terser for es', () => {
+    const terserEs = readFile('dist/terser/my-lib-custom-filename.js')
+    expect(terserEs).not.toMatch('debugger')
+  })
+
+  test('pure annotations are not removed by terser for es', () => {
+    const terserEs = readFile('dist/terser/my-lib-custom-filename.js')
+    expect(terserEs).toMatch(/[@#]__PURE__/)
+  })
+
+  test('pure annotations are removed by terser for non-es output', () => {
+    const terserIife = readFile('dist/terser/my-lib-custom-filename.iife.js')
+    expect(terserIife).not.toMatch(/[@#]__PURE__/)
+  })
+
   test('single entry with css', () => {
     const css = readFile('dist/css-single-entry/test-my-lib.css')
     const js = readFile('dist/css-single-entry/test-my-lib.js')

playground/lib/__tests__/serve.ts

@@ -108,6 +108,12 @@ export async function serve(): Promise<{ close(): Promise<void> }> {
       configFile: path.resolve(__dirname, '../vite.css-code-split.config.js'),
     })
 
+    await build({
+      root: rootDir,
+      logLevel: 'warn', // output esbuild warns
+      configFile: path.resolve(__dirname, '../vite.terser.config.js'),
+    })
+
     // start static file server
     const serve = sirv(path.resolve(rootDir, 'dist'))
     const httpServer = http.createServer((req, res) => {

playground/lib/src/main.js

@@ -1,4 +1,4 @@
-export default function myLib(sel) {
+export default /* @__PURE__ */ Object.assign(function myLib(sel) {
   // Force esbuild spread helpers (https://github.com/evanw/esbuild/issues/951)
   console.log({ ...'foo' })
 
@@ -9,7 +9,10 @@ export default function myLib(sel) {
 
   // make sure umd helper has been moved to the right position
   console.log(`amd function(){ "use strict"; }`)
-}
+
+  // eslint-disable-next-line no-debugger
+  debugger
+})
 
 // For triggering unhandled global esbuild helpers in previous regex-based implementation for injection
 ;(function () {})()?.foo

playground/lib/vite.terser.config.js

@@ -0,0 +1,24 @@
+import path from 'node:path'
+import { defineConfig } from 'vite'
+import baseConfig from './vite.config'
+
+export default defineConfig({
+  ...baseConfig,
+  build: {
+    ...baseConfig.build,
+    minify: 'terser',
+    terserOptions: {
+      compress: {
+        drop_debugger: true,
+      },
+    },
+    outDir: 'dist/terser',
+    lib: {
+      ...baseConfig.build.lib,
+      entry: path.resolve(__dirname, 'src/main.js'),
+      formats: ['es', 'iife'],
+    },
+  },
+  plugins: [],
+  cacheDir: 'node_modules/.vite-terser',
+})