diff --git a/packages/vite/src/node/plugins/esbuild.ts b/packages/vite/src/node/plugins/esbuild.ts
index ffd9dbcf287884..f3a0b1d222fc8a 100644
--- a/packages/vite/src/node/plugins/esbuild.ts
+++ b/packages/vite/src/node/plugins/esbuild.ts
@@ -29,7 +29,7 @@ const debug = createDebugger('vite:esbuild')
 // IIFE content looks like `var MyLib = function() {`.
 // Spaces are removed and parameters are mangled when minified
 const IIFE_BEGIN_RE =
-  /(?:const|var)\s+\S+\s*=\s*function\([^()]*\)\s*\{\s*"use strict";/
+  /(?:const|var)\s+\S+\s*=\s*\(?function\([^()]*\)\s*\{\s*"use strict";/
 
 const validExtensionRE = /\.\w+$/
 const jsxExtensionsRE = /\.(?:j|t)sx\b/