get it working

pnpm-lock.yaml

@@ -4919,8 +4919,8 @@ packages:
       loose-envify: 1.4.0
     dev: false
 
-  /scule@1.0.0:
-    resolution: {integrity: sha512-4AsO/FrViE/iDNEPaAQlb77tf0csuq27EsVpy6ett584EcRTp6pTDLoGWVxCD77y5iU5FauOvhsI4o1APwPoSQ==}
+  /scule@1.3.0:
+    resolution: {integrity: sha512-6FtHJEvt+pVMIB9IBY+IcCJ6Z5f1iQnytgyfKMhDKgmzYG+TeH/wx1y3l27rshSbLiSanrR9ffZDrEsmjlQF2g==}
     dev: false
 
   /semver-diff@4.0.0:
@@ -5550,7 +5550,7 @@ packages:
       mlly: 1.4.2
       pathe: 1.1.1
       pkg-types: 1.0.3
-      scule: 1.0.0
+      scule: 1.3.0
       strip-literal: 1.3.0
       unplugin: 1.5.0
     transitivePeerDependencies:

src/core/builders/vite/index.ts

@@ -19,6 +19,7 @@ import {
 } from '~/core/utils/virtual-modules';
 import { Hookable } from 'hookable';
 import { toArray } from '~/core/utils/arrays';
+import { safeVarName } from '~/core/utils/strings';
 
 export async function createViteBuilder(
   wxtConfig: ResolvedConfig,
@@ -87,14 +88,22 @@ export async function createViteBuilder(
       plugins.push(wxtPlugins.cssEntrypoints(entrypoint, wxtConfig));
     }
 
+    const iifeReturnValueName = safeVarName(entrypoint.name);
     const libMode: vite.UserConfig = {
       mode: wxtConfig.mode,
       plugins,
+      esbuild: {
+        // Add a footer with the returned value so it can return values to `scripting.executeScript`
+        // Footer is added apart of esbuild to make sure it's not minified. It
+        // get's removed if added to `build.rollupOptions.output.footer`
+        // See https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/executeScript#return_value
+        footer: iifeReturnValueName + ';',
+      },
       build: {
         lib: {
           entry,
           formats: ['iife'],
-          name: '_',
+          name: iifeReturnValueName,
           fileName: entrypoint.name,
         },
         rollupOptions: {

src/core/utils/strings.ts

@@ -5,6 +5,14 @@ export function kebabCaseAlphanumeric(str: string): string {
     .replace(/\s+/g, '-'); // Replace spaces with hyphens
 }
 
+/**
+ * Return a safe variable name for a given string.
+ */
+export function safeVarName(str: string): string {
+  // _ prefix to ensure it doesn't start with a number
+  return '_' + kebabCaseAlphanumeric(str).replace('-', '_');
+}
+
 /**
  * Removes import statements from the top of a file. Keeps import.meta and inline, async `import()`
  * calls.

src/virtual/content-script-isolated-world-entrypoint.ts

@@ -17,7 +17,8 @@ const result = (async () => {
   }
 })();
 
-// Return the main function's result to the background when executed via the scripting API.
+// Return the main function's result to the background when executed via the
+// scripting API. Default export causes the IIFE to return a value.
 // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/executeScript#return_value
 // Tested on both Chrome and Firefox
-result;
+export default result;

src/virtual/content-script-main-world-entrypoint.ts

@@ -14,7 +14,8 @@ const result = (async () => {
   }
 })();
 
-// Return the main function's result to the background when executed via the scripting API.
+// Return the main function's result to the background when executed via the
+// scripting API. Default export causes the IIFE to return a value.
 // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/executeScript#return_value
 // Tested on both Chrome and Firefox
-result;
+export default result;

src/virtual/unlisted-script-entrypoint.ts

@@ -13,7 +13,8 @@ const result = (async () => {
   }
 })();
 
-// Return the main function's result to the background when executed via the scripting API.
+// Return the main function's result to the background when executed via the
+// scripting API. Default export causes the IIFE to return a value.
 // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/executeScript#return_value
 // Tested on both Chrome and Firefox
-result;
+export default result;