Msw: Fallback handler

[danielstreit]

Is your feature request related to a problem? Please describe.
When writing tests, I would like to ensure that no external calls are made. If a request is made and there is no matching path handler registered in msw, I would like to handle the request and throw an error so that the test fails.

Describe the solution you'd like
I'd like a fallback handler that would allow me to handle unmatched requests. Maybe something like this?

import { rest, setupWorker } from 'msw';

export const worker = setupWorker(
    rest.fallback((req, res, ctx) => {
        throw new Error(`No matching path found for ${req.url}`);
    });
);

I would probably only register this handler in the test environment. Otherwise, I'd probably use the default fallback, and let the request go to the actual server.

Describe alternatives you've considered
I believe I could implement this with a path of *, but I would have to add multiple handlers for each method (rest.get('*', fallback), rest.post('*', fallback), etc.).

Maybe there is already a better way of doing this?

[kettanaito]

Hey, @danielstreit! Thanks for reaching out with this suggestion.

While a generic fallback handler is an interesting topic, it seems that what you describe can be solved by #191?

Imagine having an API like this in your tests:

const server = setupServer(/* your handlers */)

// The API is to be discussed
beforeAll(() => server.listen({ onUnhandledRequests: 'error' })

We are planning to ship this feature in the next release of the library. Would this work for you?

[danielstreit]

Yes, that would handle the use case very well.

[kettanaito]

Glad to hear that!
I'm closing this issue in favor of #191 then.

Regarding the generic fallback handler, I can still recommend a solution using the current API. Consider this:

// This is your custom request handler function!
// That's an advanced technique, and it didn't make it to the docs just yet.
const withFallback = (resolver) => {
  return {
    predicate() {
      // Match all requests, regardless of URL, method, etc.
      return true
    },
    resolver,
  }
}

setupWorker(
  rest.get('/user', (req, res, ctx) => {...}),
  withFallback((req, res, ctx) => {
    // Handle any requests that didn't match the existing handlers.
    // Throw exceptions, or return a mocked response.
  })
)