instanceof filtering with multiple msw versions

[meesvandongen]

I have recently come across an issue where we had multiple versions of MSW in a monorepo:

• One for the API client project, which defines the handlers which can be mocked.
• One for the e2e test, which uses @msw/playwright
• One for the app project.

@msw/playwright does this at some point:

const handlers = server.listHandlers().filter((handler) => {
  return handler instanceof RequestHandler
})

If there are multiple RequestHandler declarations due to different versions being resolved, this check will fail even if the handler could be used as a request handler.

The behavior then is that it seems like msw is not intercepting anything; in my case this resulted in responses with status code 500 and an empty body.

I don't think this is necessarily a problem on msw side; this discussion is simply to document the behavior. Users of monorepo's should probably use something (e.g. PNPM Catalogs) to prevent this type of issue.

[kettanaito]

Hi, @meesvandongen. Thanks for raising this.

Perhaps we can help with this kind of issues as well by setting a loose peer dependency version on packages like @msw/playwright so they end up using the msw version you have installed.

A pull request would be welcome here!