Unable to determine whether req.body was sent as valid JSON

Hi, big fan of msw. But I’m experiencing the following issue:

Describe the bug

I want to return a different response in my resolver depending on whether req.body is valid JSON or not. This is to allow closer imitation of our backend API within our tests.

Previously my test passed despite sending invalid JSON as I always returned a success response, therefore I would like to return an error response from my mock API if the JSON sent is invalid. I believe this is the correct approach as the Request assertions documentation states that we should not be asserting on the body directly and instead include failure scenarios.

I’m currently unable to validate whether the JSON sent is valid as parseBody automatically parses JSON for us or will return the raw object if not parsable, therefore, the following two requests will have the same body in the request resolver.

Valid JSON Request

fetch("/example", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  body: JSON.stringify({ foo: 'bar' }),
});

Invalid JSON Request

fetch("/example", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  body: { foo: 'bar' },
});

Resolver

rest.post("/example", (req, res, ctx) => {
  const body = req.body; // Will always be the raw object

  // Shamelessly stolen from https://stackoverflow.com/questions/9804777/how-to-test-if-a-string-is-json-or-not
  function isValidJson(jsonInput) {
    try {
      JSON.parse(jsonInput);
    } catch (e) {
      return false;
    }
    return true;
  }
  
  const bodyWasValid = isValidJSON(req.body); // will always be false

  return res(ctx.json({ bodyWasValid }));
});

In this scenario isValidJSON would always return as false because req.body is always the raw object, I understand returning the raw object is a convenience helper and is a desired behaviour but it makes it impossible to imitate our backend API.

Environment

• msw: ^0.21.3
• nodejs: 12.18.3
• npm: 6.14.8

To Reproduce

Use the above resolver with the above payloads.

Expected behavior

As mentioned above I recognise the automatic parsing of req.body is intentional and would be a breaking change if removed.

My initial thoughts should be that req.body should be ‘[object Object]’ if sent as an object that is not valid JSON. If the body was sent as valid JSON then it should remain as the parsed object.

[kettanaito]

The fix on the whatwg-fetch side has been published: https://github.com/github/fetch/releases/tag/v3.6.0. I suggest updating that dependency and checking if the issue is gone.

[kettanaito]

I see little value in keeping this issue open: there is nothing to be addressed on the MSW side. The issue is in the whatwg-fetch polyfill as it handles a non-stringified JSON body differently than window.fetch in a browser (see #856). As the status of that polyfill is “not maintained”, I can suggest you switching to a different polyfill in your tests.

You can still track the resolution progress with whatwg-fetch in this pull request: https://github.com/github/fetch/pull/881.