fix: use browser field if likely esm (fixes #9652)

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

@@ -3,6 +3,7 @@ import path from 'node:path'
 import colors from 'picocolors'
 import type { PartialResolvedId } from 'rollup'
 import { resolve as _resolveExports } from 'resolve.exports'
+import { hasESMSyntax } from 'mlly'
 import type { Plugin } from '../plugin'
 import {
   DEFAULT_EXTENSIONS,
@@ -850,25 +851,22 @@ export function resolvePackageEntry(
         ) {
           // if both are present, we may have a problem: some package points both
           // to ESM, with "module" targeting Node.js, while some packages points
-          // "module" to browser ESM and "browser" to UMD.
+          // "module" to browser ESM and "browser" to UMD/IIFE.
           // the heuristics here is to actually read the browser entry when
-          // possible and check for hints of UMD. If it is UMD, prefer "module"
+          // possible and check for hints of ESM. If it is not ESM, prefer "module"
           // instead; Otherwise, assume it's ESM and use it.
           const resolvedBrowserEntry = tryFsResolve(
             path.join(dir, browserEntry),
             options
           )
           if (resolvedBrowserEntry) {
             const content = fs.readFileSync(resolvedBrowserEntry, 'utf-8')
-            if (
-              (/typeof exports\s*==/.test(content) &&
-                /typeof module\s*==/.test(content)) ||
-              /module\.exports\s*=/.test(content)
-            ) {
-              // likely UMD or CJS(!!! e.g. firebase 7.x), prefer module
-              entryPoint = data.module
-            } else {
+            if (hasESMSyntax(content)) {
+              // likely ESM, prefer browser
               entryPoint = browserEntry
+            } else {
+              // non-ESM, UMD or IIFE or CJS(!!! e.g. firebase 7.x), prefer module
+              entryPoint = data.module
             }
           }
         } else {

playground/resolve/__tests__/resolve.spec.ts

@@ -96,6 +96,10 @@ test('Resolve module field if browser field is likely UMD or CJS', async () => {
   expect(await page.textContent('.browser-module2')).toMatch('[success]')
 })
 
+test('Resolve module field if browser field is likely IIFE', async () => {
+  expect(await page.textContent('.browser-module3')).toMatch('[success]')
+})
+
 test('css entry', async () => {
   expect(await page.textContent('.css')).toMatch('[success]')
 })

playground/resolve/browser-module-field3/index.js

@@ -0,0 +1 @@
+export default '[success] this should run in browser'

playground/resolve/browser-module-field3/index.web.js

@@ -0,0 +1,7 @@
+var browserModuleField3 = (function () {
+  'use strict'
+
+  var main = '[fail] this should not run in the browser'
+
+  return main
+})()

playground/resolve/browser-module-field3/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "resolve-browser-module-field3",
+  "private": true,
+  "version": "1.0.0",
+  "module": "index.js",
+  "browser": "index.web.js"
+}

playground/resolve/index.html

@@ -82,6 +82,9 @@ <h2>Resolve browser field even if module field exists</h2>
 <h2>Resolve module field if browser field is likely UMD or CJS</h2>
 <p class="browser-module2">fail</p>
 
+<h2>Resolve module field if browser field is likely IIFE</h2>
+<p class="browser-module3">fail</p>
+
 <h2>Don't resolve to the `module` field if the importer is a `require` call</h2>
 <p class="require-pkg-with-module-field">fail</p>
 
@@ -225,6 +228,9 @@ <h2>resolve package that contains # in path</h2>
   import browserModule2 from 'resolve-browser-module-field2'
   text('.browser-module2', browserModule2)
 
+  import browserModule3 from 'resolve-browser-module-field3'
+  text('.browser-module3', browserModule3)
+
   import { msg as requireButWithModuleFieldMsg } from 'require-pkg-with-module-field'
   text('.require-pkg-with-module-field', requireButWithModuleFieldMsg)
 

playground/resolve/package.json

@@ -16,6 +16,7 @@
     "resolve-browser-field": "link:./browser-field",
     "resolve-browser-module-field1": "link:./browser-module-field1",
     "resolve-browser-module-field2": "link:./browser-module-field2",
+    "resolve-browser-module-field3": "link:./browser-module-field3",
     "resolve-custom-condition": "link:./custom-condition",
     "resolve-custom-main-field": "link:./custom-main-field",
     "resolve-exports-env": "link:./exports-env",