React.Lazy import using Fullstack Dev Server & "bun start" not working correctly

[walidsj]

What version of Bun is running?

1.2.10+db2e7d7f7

What platform is your computer?

MINGW64_NT-10.0-26100 3.4.6.x86_64 x86_64

What steps can reproduce the bug?

I'm using react-router with createBrowserRouter and React.lazy() to implement lazy-loaded routes. However, when running with Bun, the code for lazy-loaded components is not split into separate chunks and is instead bundled and loaded eagerly on initial load.

Steps to reproduce the behavior:

1. Define routes using createBrowserRouter in App.tsx with React.lazy():

   import { createBrowserRouter, Outlet, RouterProvider } from 'react-router';
   import { Suspense, lazy } from 'react';
   
   const PageComponent = lazy(() => import('./pages/admin'));
   
   function LoadingComponent() {
      return <div>Loading...</div>
   }
   
   function SuspenseWrapper() {
      return (
         <Suspense fallback={<LoadingComponent />}>
            <Outlet />
         </Suspense>
     );
   }
   
   
   const router = createBrowserRouter([
     { 
       Component: SuspenseWrapper,
       children: [
          {
             path: "/admin",
             Component: PageComponent
          }
       ]
     },
   ]);
   
   <RouterProvider router={router} />
   

2. Run bun start where in the scripts NODE_ENV=production bun index.tsx
3. Both index.tsx and index.html remain the same as official template Bun + React (TailwindCSS + ShadcnUI)

What is the expected behavior?

This works correctly when running the same code using Node.js or Vite, where lazy-loaded components are split and loaded only when needed.

What do you see instead?

However, when running the app with Bun, as a result, React.lazy() is ineffective — the entire codebase is loaded at once. There is no error message, but the expected component never shows up.

Additional information

No errors are shown in console. The issue seems related to Bun's handling spliting code and load of dynamic imports or React.lazy() behavior. Static imports work fine.

[RiskyMH]

I'm not sure about the dev server, but for the actually built one with the build.tsx it does split correctly (where imo it matters way more). I think for dev server it tries to do everything at once for peek hmr perf, but clearly it is too eger.