fix(html): move importmap before module scripts

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

@@ -7,6 +7,7 @@ import type {
   SourceMapInput
 } from 'rollup'
 import MagicString from 'magic-string'
+import colors from 'picocolors'
 import type {
   AttributeNode,
   CompilerError,
@@ -54,6 +55,10 @@ const inlineImportRE =
   /(?<!(?<!\.\.)\.)\bimport\s*\(("([^"]|(?<=\\)")*"|'([^']|(?<=\\)')*')\)/g
 const htmlLangRE = /\.(html|htm)$/
 
+const importMapRE =
+  /[ \t]*<script[^>]*type\s*=\s*["']?importmap["']?[^>]*>.*?<\/script>/is
+const moduleScriptRE = /[ \t]*<script[^>]*type\s*=\s*["']?module["']?[^>]*>/is
+
 export const isHTMLProxy = (id: string): boolean => htmlProxyRE.test(id)
 
 export const isHTMLRequest = (request: string): boolean =>
@@ -225,6 +230,8 @@ function handleParseError(
  */
 export function buildHtmlPlugin(config: ResolvedConfig): Plugin {
   const [preHooks, postHooks] = resolveHtmlTransforms(config.plugins)
+  preHooks.unshift(preImportMapHook(config))
+  postHooks.push(postImportMapHook())
   const processedHtml = new Map<string, string>()
   const isExcludedUrl = (url: string) =>
     url.startsWith('#') ||
@@ -796,6 +803,51 @@ export type IndexHtmlTransform =
       transform: IndexHtmlTransformHook
     }
 
+export function preImportMapHook(
+  config: ResolvedConfig
+): IndexHtmlTransformHook {
+  return (html, ctx) => {
+    const importMapIndex = html.match(importMapRE)?.index
+    if (importMapIndex === undefined) return
+
+    const moduleScriptIndex = html.match(moduleScriptRE)?.index
+    if (moduleScriptIndex === undefined) return
+
+    if (moduleScriptIndex < importMapIndex) {
+      const relativeHtml = normalizePath(
+        path.relative(config.root, ctx.filename)
+      )
+      config.logger.warnOnce(
+        colors.yellow(
+          colors.bold(
+            `(!) <script type="importmap"> should come before <script type="module"> in /${relativeHtml}`
+          )
+        )
+      )
+    }
+  }
+}
+
+/**
+ * Move importmap before the first module script
+ */
+export function postImportMapHook(): IndexHtmlTransformHook {
+  return (html) => {
+    if (!moduleScriptRE.test(html)) return
+
+    let importMap: string | undefined
+    html = html.replace(importMapRE, (match) => {
+      importMap = match
+      return ''
+    })
+    if (importMap) {
+      html = html.replace(moduleScriptRE, (match) => `${importMap}\n${match}`)
+    }
+
+    return html
+  }
+}
+
 export function resolveHtmlTransforms(
   plugins: readonly Plugin[]
 ): [IndexHtmlTransformHook[], IndexHtmlTransformHook[]] {

packages/vite/src/node/server/middlewares/indexHtml.ts

@@ -11,6 +11,8 @@ import {
   applyHtmlTransforms,
   assetAttrsConfig,
   getScriptInfo,
+  postImportMapHook,
+  preImportMapHook,
   resolveHtmlTransforms,
   traverseHtml
 } from '../../plugins/html'
@@ -43,12 +45,22 @@ export function createDevHtmlTransformFn(
 ): (url: string, html: string, originalUrl: string) => Promise<string> {
   const [preHooks, postHooks] = resolveHtmlTransforms(server.config.plugins)
   return (url: string, html: string, originalUrl: string): Promise<string> => {
-    return applyHtmlTransforms(html, [...preHooks, devHtmlHook, ...postHooks], {
-      path: url,
-      filename: getHtmlFilename(url, server),
-      server,
-      originalUrl
-    })
+    return applyHtmlTransforms(
+      html,
+      [
+        preImportMapHook(server.config),
+        ...preHooks,
+        devHtmlHook,
+        ...postHooks,
+        postImportMapHook()
+      ],
+      {
+        path: url,
+        filename: getHtmlFilename(url, server),
+        server,
+        originalUrl
+      }
+    )
   }
 }
 

playground/external/__tests__/external.spec.ts

@@ -1,5 +1,11 @@
 import { describe, expect, test } from 'vitest'
-import { isBuild, page } from '~utils'
+import { browserLogs, isBuild, page } from '~utils'
+
+test('importmap', () => {
+  expect(browserLogs).not.toContain(
+    'An import map is added after module script load was triggered.'
+  )
+})
 
 describe.runIf(isBuild)('build', () => {
   test('should externalize imported packages', async () => {

playground/html/__tests__/html.spec.ts

@@ -1,5 +1,13 @@
 import { beforeAll, describe, expect, test } from 'vitest'
-import { editFile, getColor, isBuild, isServe, page, viteTestUrl } from '~utils'
+import {
+  browserLogs,
+  editFile,
+  getColor,
+  isBuild,
+  isServe,
+  page,
+  viteTestUrl
+} from '~utils'
 
 function testPage(isNested: boolean) {
   test('pre transform', async () => {
@@ -242,3 +250,9 @@ describe.runIf(isServe)('invalid', () => {
     expect(content).toBeTruthy()
   })
 })
+
+test('importmap', () => {
+  expect(browserLogs).not.toContain(
+    'An import map is added after module script load was triggered.'
+  )
+})

playground/html/vite.config.js

@@ -160,6 +160,25 @@ ${
           }
         ]
       }
+    },
+    {
+      name: 'head-prepend-importmap',
+      transformIndexHtml() {
+        return [
+          {
+            tag: 'script',
+            attrs: { type: 'importmap' },
+            children: `
+              {
+                "imports": {
+                  "vue": "https://unpkg.com/vue@3.2.0/dist/vue.runtime.esm-browser.js"
+                }
+              }
+            `,
+            injectTo: 'head'
+          }
+        ]
+      }
     }
   ]
 }