fix(plugin-legacy): legacy fallback for dynamic import

packages/plugin-legacy/README.md

@@ -122,6 +122,13 @@ export default {
   }
   ```
 
+## Dynamic Import
+
+The legacy plugin offers a way to use native `import()` in the modern build while falling back to the legacy build in browsers with native ESM but without dynamic import support (e.g. Legacy Edge). This feature works by injecting a runtime check and loading the legacy bundle with SystemJs runtime if needed. There are the following drawbacks:
+
+- Modern bundle is downloaded in all ESM browsers
+- Modern bundle throws `SyntaxError` in browsers without dynamic import
+
 ## Polyfill Specifiers
 
 Polyfill specifier strings for `polyfills` and `modernPolyfills` can be either of the following:
@@ -147,12 +154,13 @@ export default {
 
 ## Content Security Policy
 
-The legacy plugin requires inline scripts for [Safari 10.1 `nomodule` fix](https://gist.github.com/samthor/64b114e4a4f539915a95b91ffd340acc) and SystemJS initialization. If you have a strict CSP policy requirement, you will need to [add the corresponding hashes to your `script-src` list](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src#unsafe_inline_script):
+The legacy plugin requires inline scripts for [Safari 10.1 `nomodule` fix](https://gist.github.com/samthor/64b114e4a4f539915a95b91ffd340acc), SystemJS initialization, and dynamic import fallback. If you have a strict CSP policy requirement, you will need to [add the corresponding hashes to your `script-src` list](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src#unsafe_inline_script):
 
 - `MS6/3FCg4WjP9gwgaBGwLpRCY6fZBgwmhVCdrPrNf3E=`
 - `tQjf8gvb2ROOMapIxFvFAYBeUJ0v1HCbOcSmDNXGtDo=`
+- `T9h4ixy0FtNsCwAyTfBtIY6uV5ZhMeNQIlL42GAKEME=`
 
-These values can also be retrived via
+These values can also be retrieved via
 
 ```js
 const { cspHashes } = require('@vitejs/plugin-legacy')

packages/plugin-legacy/index.js

@@ -15,8 +15,12 @@ const loadBabel = () => babel || (babel = require('@babel/standalone'))
 // DO NOT ALTER THIS CONTENT
 const safari10NoModuleFix = `!function(){var e=document,t=e.createElement("script");if(!("noModule"in t)&&"onbeforeload"in t){var n=!1;e.addEventListener("beforeload",(function(e){if(e.target===t)n=!0;else if(!e.target.hasAttribute("nomodule")||!n)return;e.preventDefault()}),!0),t.type="module",t.src=".",e.head.appendChild(t),t.remove()}}();`
 
+const legacyPolyfillId = 'vite-legacy-polyfill'
 const legacyEntryId = 'vite-legacy-entry'
 const systemJSInlineCode = `System.import(document.getElementById('${legacyEntryId}').getAttribute('data-src'))`
+const dynamicFallbackInlineCode = `!function(){try{new Function("m","return import(m)")}catch(o){console.warn("vite: loading legacy build because dynamic import is unsupported, syntax error above should be ignored");var e=document.getElementById("${legacyPolyfillId}"),n=document.createElement("script");n.src=e.src,n.onload=function(){${systemJSInlineCode}},document.body.appendChild(n)}}();`
+
+const blankDynamicImport = `import('data:text/javascript;base64,Cg==');`
 
 const legacyEnvVarMarker = `__VITE_IS_LEGACY__`
 
@@ -31,6 +35,7 @@ function viteLegacyPlugin(options = {}) {
   let config
   const targets = options.targets || 'defaults'
   const genLegacy = options.renderLegacyChunks !== false
+  const genDynamicFallback = genLegacy
 
   const debugFlag = process.env.DEBUG
   const isDebug = debugFlag === 'vite:*' || debugFlag === 'vite:legacy'
@@ -76,9 +81,6 @@ function viteLegacyPlugin(options = {}) {
       if (!config.build) {
         config.build = {}
       }
-      if (genLegacy) {
-        config.build.polyfillDynamicImport = true
-      }
     }
   }
 
@@ -123,7 +125,7 @@ function viteLegacyPlugin(options = {}) {
       }
 
       // legacy bundle
-      if (legacyPolyfills.size) {
+      if (legacyPolyfills.size || genDynamicFallback) {
         if (!legacyPolyfills.has('es.promise')) {
           // check if the target needs Promise polyfill because SystemJS relies
           // on it
@@ -228,28 +230,31 @@ function viteLegacyPlugin(options = {}) {
           detectPolyfills(raw, { esmodules: true }, modernPolyfills)
         }
 
+        const ms = new MagicString(raw)
+
+        if (genDynamicFallback && chunk.isEntry) {
+          ms.prepend(blankDynamicImport)
+        }
+
         if (raw.includes(legacyEnvVarMarker)) {
           const re = new RegExp(legacyEnvVarMarker, 'g')
-          if (config.build.sourcemap) {
-            const s = new MagicString(raw)
-            let match
-            while ((match = re.exec(raw))) {
-              s.overwrite(
-                match.index,
-                match.index + legacyEnvVarMarker.length,
-                `false`
-              )
-            }
-            return {
-              code: s.toString(),
-              map: s.generateMap({ hires: true })
-            }
-          } else {
-            return raw.replace(re, `false`)
+          let match
+          while ((match = re.exec(raw))) {
+            ms.overwrite(
+              match.index,
+              match.index + legacyEnvVarMarker.length,
+              `false`
+            )
           }
         }
 
-        return null
+        if (config.build.sourcemap) {
+          return {
+            code: ms.toString(),
+            map: ms.generateMap({ hires: true })
+          }
+        }
+        return ms.toString()
       }
 
       if (!genLegacy) {
@@ -358,6 +363,7 @@ function viteLegacyPlugin(options = {}) {
           tag: 'script',
           attrs: {
             nomodule: true,
+            id: legacyPolyfillId,
             src: `${config.base}${legacyPolyfillFilename}`
           },
           injectTo: 'body'
@@ -392,6 +398,16 @@ function viteLegacyPlugin(options = {}) {
         )
       }
 
+      // 5. inject dynamic import fallback entry
+      if (genDynamicFallback && legacyPolyfillFilename && legacyEntryFilename) {
+        tags.push({
+          tag: 'script',
+          attrs: { type: 'module' },
+          children: dynamicFallbackInlineCode,
+          injectTo: 'head'
+        })
+      }
+
       return {
         html,
         tags
@@ -639,5 +655,6 @@ viteLegacyPlugin.default = viteLegacyPlugin
 
 viteLegacyPlugin.cspHashes = [
   createHash('sha256').update(safari10NoModuleFix).digest('base64'),
-  createHash('sha256').update(systemJSInlineCode).digest('base64')
+  createHash('sha256').update(systemJSInlineCode).digest('base64'),
+  createHash('sha256').update(dynamicFallbackInlineCode).digest('base64')
 ]