Improve error message when file system route files have no exports

[buttercat1791]

I encountered a bug while attempting to write a middleware while using filesystem-based routing that caused requests to API routes to fail.

My app defined a middleware to run on all routes in the file routes/_middleware.ts. When an island attempted to call an API endpoint under routes/api/, the request failed with the following error on the server:

Error: Expected a route, middleware, layout or error template, but couldn't find relevant exports in: undefined

I was able to create a minimal reproduction by simply adding a _middleware.ts file to the Fresh starter app (GitHub).

A workaround is possible by switching to programmatic routing rather than filesystem routing (example).

I am using Fresh v2.2.0 and Deno v2.5.6.

[marvinhagemeister]

File system based files are expected to have exports. The error message could be maybe rephrased a little to make this more clear.

To resolve this issue make sure you actually export your middleware.

  import { define } from "../utils.ts";
  
- define.middleware(async (ctx) => {
+ export default define.middleware(async (ctx) => {
    console.log(`Request to ${ctx.req}`);
    return await ctx.next();
  });

[buttercat1791]

Thanks for the reply!

Adding the export makes the reproduction example work, but it may not be an accurate reproduction.

In the project in which I noticed the issue, I had a routes/_middleware.ts defined as such:

import { define } from "@/utils.ts";
import { accessControlMiddlewareHandler } from "@/features/auth/access-control-middleware.ts";

const accessControlMiddleware = define.middleware(
  accessControlMiddlewareHandler,
);

export default [accessControlMiddleware];

The accessControlMiddlewareHandler import has a signature of:

export async function accessControlMiddlewareHandler(ctx: Context<State>): Promise<Response>

I've updated my reproduction to better illustrate this case.

[marvinhagemeister]

You cannot export a middleware as a default export in routes. Routes have a different format.

// Page handlers are exported as `handler`
export const handler = define.handlers(...)

// Default export is expected to be a component
export default define.page<typeof handlers>(props => {
  return <h1>Hello world</h1>
})

Applying this to your reproduction:

  import { define } from "../../utils.ts";
  
- export default define.handlers({
+ export const handler = define.handlers({
    async POST(ctx) {
      // ...snip
    },
  });