fix: Add compatibility mode option to disable Content-Type header checking

[alh-solidatus]

This was a breaking change with v3 that's hard to work around when you have broken APIs. I've added a config option to allow use of this library where it was possible to do so before.

Add relevant information to migration guide so users are aware of the breaking change.

[alh-solidatus]

@rexxars Is this something you would consider merging?

[rexxars]

Hi, apologies for the late response.

No, unfortunately I am not willing to add this to the core module since it does not align with the spec, and the implementations in browsers. Both Chrome and Firefox will reject the request if it has the wrong Content-Type, for example.

However, you should be able to work around this by specifying a custom fetch function that adds/modifies the Content-Type before returning (untested code, but think this should work):

const es = new EventSource("/my-non-spec-sse-endpoint", {
  fetch: async (input, init) => {
    const response = await fetch(input, init);
    // Clone the original response, overriding the content-type with a spec-conforming one
    const newHeaders = new Headers(response.headers);
    newHeaders.set("Content-Type", "text/event-stream");
    return new Response(response.body, {
      status: response.status,
      statusText: response.statusText,
      headers: newHeaders,
    });
  },
});

es.addEventListener("open", () => {
  /* … */
});

[rexxars]

Update: I tested the above code to ensure it works, and it does :)

Added it to the migration guide for future reference - thank you for raising!