fix: restore dynamic-import-polyfill

docs/config/index.md

@@ -480,6 +480,23 @@ export default async ({ command, mode }) => {
 
   Note the build will fail if the code contains features that cannot be safely transpiled by esbuild. See [esbuild docs](https://esbuild.github.io/content-types/#javascript) for more details.
 
+### build.polyfillDynamicImport
+
+- **Type:** `boolean`
+- **Default:** `false`
+
+  Whether to automatically inject [dynamic import polyfill](https://github.com/GoogleChromeLabs/dynamic-import-polyfill).
+
+  If set to true, the polyfill is auto injected into the proxy module of each `index.html` entry. If the build is configured to use a non-html custom entry via `build.rollupOptions.input`, then it is necessary to manually import the polyfill in your custom entry:
+
+  ```js
+  import 'vite/dynamic-import-polyfill'
+  ```
+
+  When using [`@vitejs/plugin-legacy`](https://github.com/vitejs/vite/tree/main/packages/plugin-legacy), the plugin sets this option to `true` automatically.
+
+  Note: the polyfill does **not** apply to [Library Mode](/guide/build#library-mode). If you need to support browsers without native dynamic import, you should probably avoid using it in your library.
+
 ### build.outDir
 
 - **Type:** `string`

docs/guide/backend-integration.md

@@ -20,6 +20,13 @@ Or you can follow these steps to configure it manually:
    }
    ```
 
+   If you use [`@vitejs/plugin-legacy`](https://github.com/vitejs/vite/tree/main/packages/plugin-legacy) or manually enable the [`build.dynamicImportPolyfill` option](/config/#build-polyfilldynamicimport), remember to add the [dynamic import polyfill](/config/#build-polyfilldynamicimport) to your entry, since it will no longer be auto-injected:
+
+   ```js
+   // add the beginning of your app entry
+   import 'vite/dynamic-import-polyfill'
+   ```
+
 2. For development, inject the following in your server's HTML template (substitute `http://localhost:3000` with the local URL Vite is running at):
 
    ```html

packages/plugin-legacy/index.js

@@ -65,6 +65,20 @@ function viteLegacyPlugin(options = {}) {
     })
   }
 
+  /**
+   * @type {import('vite').Plugin}
+   */
+  const legacyConfigPlugin = {
+    name: 'legacy-config',
+
+    config(config) {
+      if (!config.build) {
+        config.build = {}
+      }
+      config.build.polyfillDynamicImport = true
+    }
+  }
+
   /**
    * @type {import('vite').Plugin}
    */
@@ -398,7 +412,12 @@ function viteLegacyPlugin(options = {}) {
     }
   }
 
-  return [legacyGenerateBundlePlugin, legacyPostPlugin, legacyEnvPlugin]
+  return [
+    legacyConfigPlugin,
+    legacyGenerateBundlePlugin,
+    legacyPostPlugin,
+    legacyEnvPlugin
+  ]
 }
 
 /**

packages/vite/src/node/build.ts

@@ -64,7 +64,7 @@ export interface BuildOptions {
   /**
    * whether to inject dynamic import polyfill.
    * Note: does not apply to library mode.
-   * @deprecated the dynamic import polyfill has been removed
+   * @default false
    */
   polyfillDynamicImport?: boolean
   /**
@@ -194,13 +194,12 @@ export interface LibraryOptions {
 
 export type LibraryFormats = 'es' | 'cjs' | 'umd' | 'iife'
 
-export type ResolvedBuildOptions = Required<
-  Omit<BuildOptions, 'base' | 'polyfillDynamicImport'>
->
+export type ResolvedBuildOptions = Required<Omit<BuildOptions, 'base'>>
 
 export function resolveBuildOptions(raw?: BuildOptions): ResolvedBuildOptions {
   const resolved: ResolvedBuildOptions = {
     target: 'modules',
+    polyfillDynamicImport: false,
     outDir: 'dist',
     assetsDir: 'assets',
     assetsInlineLimit: 4096,

packages/vite/src/node/config.ts

@@ -470,25 +470,6 @@ export async function resolveConfig(
     }
   })
 
-  if (config.build?.polyfillDynamicImport) {
-    logDeprecationWarning(
-      'build.polyfillDynamicImport',
-      '"polyfillDynamicImport" has been removed. Please use @vitejs/plugin-legacy if your target browsers do not support dynamic imports.'
-    )
-  }
-
-  Object.defineProperty(resolvedBuildOptions, 'polyfillDynamicImport', {
-    enumerable: false,
-    get() {
-      logDeprecationWarning(
-        'build.polyfillDynamicImport',
-        '"polyfillDynamicImport" has been removed. Please use @vitejs/plugin-legacy if your target browsers do not support dynamic imports.',
-        new Error()
-      )
-      return false
-    }
-  })
-
   if (config.alias) {
     logDeprecationWarning('alias', 'Use "resolve.alias" instead.')
   }

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

@@ -1,12 +1,27 @@
 import { ResolvedConfig } from '..'
 import { Plugin } from '../plugin'
+import { isModernFlag } from './importAnalysisBuild'
+import path from 'path'
 
 export const polyfillId = 'vite/dynamic-import-polyfill'
 
-/**
- * @deprecated
- */
+function resolveModulePath(config: ResolvedConfig) {
+  const {
+    base,
+    build: { assetsDir }
+  } = config
+  // #2918 path.posix.join returns a wrong path when config.base is a URL
+  if (/^(https?:)?(\/\/)/i.test(base)) {
+    return `${base.replace(/\/$/, '')}/${assetsDir}/`
+  }
+  return path.posix.join(base, assetsDir, '/')
+}
+
 export function dynamicImportPolyfillPlugin(config: ResolvedConfig): Plugin {
+  const enabled = config.build.polyfillDynamicImport
+  const skip = !enabled || config.command === 'serve' || config.build.ssr
+  let polyfillString: string | undefined
+
   return {
     name: 'vite:dynamic-import-polyfill',
     resolveId(id) {
@@ -16,11 +31,114 @@ export function dynamicImportPolyfillPlugin(config: ResolvedConfig): Plugin {
     },
     load(id) {
       if (id === polyfillId) {
-        config.logger.warn(
-          `\n'vite/dynamic-import-polyfill' is no longer needed, refer to https://github.com/vitejs/vite/blob/main/packages/vite/CHANGELOG.md#230-2021-05-10`
+        if (!enabled) {
+          config.logger.warnOnce(
+            `\n'vite/dynamic-import-polyfill' is no longer needed if you target modern browsers`
+          )
+        }
+        if (skip) {
+          return ''
+        }
+        // return a placeholder here and defer the injection to renderChunk
+        // so that we can selectively skip the injection based on output format
+        if (!polyfillString) {
+          polyfillString =
+            `const p = ${polyfill.toString()};` +
+            `${isModernFlag}&&p(${JSON.stringify(resolveModulePath(config))});`
+        }
+        return polyfillString
+      }
+    },
+
+    renderDynamicImport({ format }) {
+      if (skip || format !== 'es') {
+        return null
+      }
+      if (!polyfillString) {
+        throw new Error(
+          `Vite's dynamic import polyfill is enabled but was never imported. This ` +
+            `should only happen when using custom non-html rollup inputs. Make ` +
+            `sure to add \`import "${polyfillId}"\` as the first statement in ` +
+            `your custom entry.`
         )
-        return ''
       }
+      // we do not actually return anything here because rewriting here would
+      // make it impossible to use es-module-lexer on the rendered chunks, which
+      // we need for import graph optimization in ./importAnalysisBuild.
     }
   }
 }
+
+/**
+The following polyfill function is meant to run in the browser and adapted from
+https://github.com/GoogleChromeLabs/dynamic-import-polyfill
+MIT License
+Copyright (c) 2018 uupaa and 2019 Google LLC
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+*/
+
+declare const self: any
+declare const location: any
+declare const document: any
+declare const URL: any
+declare const Blob: any
+
+function polyfill(modulePath = '.', importFunctionName = '__import__') {
+  try {
+    self[importFunctionName] = new Function('u', `return import(u)`)
+  } catch (error) {
+    const baseURL = new URL(modulePath, location)
+    const cleanup = (script: any) => {
+      URL.revokeObjectURL(script.src)
+      script.remove()
+    }
+
+    self[importFunctionName] = (url: string) =>
+      new Promise((resolve, reject) => {
+        const absURL = new URL(url, baseURL)
+
+        // If the module has already been imported, resolve immediately.
+        if (self[importFunctionName].moduleMap[absURL]) {
+          return resolve(self[importFunctionName].moduleMap[absURL])
+        }
+
+        const moduleBlob = new Blob(
+          [
+            `import * as m from '${absURL}';`,
+            `${importFunctionName}.moduleMap['${absURL}']=m;`
+          ],
+          { type: 'text/javascript' }
+        )
+
+        const script = Object.assign(document.createElement('script'), {
+          type: 'module',
+          src: URL.createObjectURL(moduleBlob),
+          onerror() {
+            reject(new Error(`Failed to import: ${url}`))
+            cleanup(script)
+          },
+          onload() {
+            resolve(self[importFunctionName].moduleMap[absURL])
+            cleanup(script)
+          }
+        })
+
+        document.head.appendChild(script)
+      })
+
+    self[importFunctionName].moduleMap = {}
+  }
+}

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

@@ -20,6 +20,7 @@ import {
   getAssetFilename
 } from './asset'
 import { isCSSRequest, chunkToEmittedCssFileMap } from './css'
+import { polyfillId } from './dynamicImportPolyfill'
 import {
   AttributeNode,
   NodeTransform,
@@ -262,6 +263,12 @@ export function buildHtmlPlugin(config: ResolvedConfig): Plugin {
         }
 
         processedHtml.set(id, s.toString())
+
+        // inject dynamic import polyfill
+        if (config.build.polyfillDynamicImport) {
+          js = `import "${polyfillId}";\n${js}`
+        }
+
         return js
       }
     },

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

@@ -204,6 +204,7 @@ export function buildImportAnalysisPlugin(config: ResolvedConfig): Plugin {
         return
       }
 
+      const isPolyfillEnabled = config.build.polyfillDynamicImport
       for (const file in bundle) {
         const chunk = bundle[file]
         // can't use chunk.dynamicImports.length here since some modules e.g.
@@ -220,7 +221,12 @@ export function buildImportAnalysisPlugin(config: ResolvedConfig): Plugin {
           if (imports.length) {
             const s = new MagicString(code)
             for (let index = 0; index < imports.length; index++) {
-              const { s: start, e: end } = imports[index]
+              const { s: start, e: end, d: dynamicIndex } = imports[index]
+              // if dynamic import polyfill is used, rewrite the import to
+              // use the polyfilled function.
+              if (isPolyfillEnabled) {
+                s.overwrite(dynamicIndex, dynamicIndex + 6, `__import__`)
+              }
               // check the chunk being imported
               const url = code.slice(start, end)
               const deps: Set<string> = new Set()