Incorrect caching of requests using URLSearchParams as body

[fugidev]

Example (adapted from https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#setting_a_body):

Fetch("https://example.com", {
  fetchOptions: {
    method: "POST",
    body: new URLSearchParams({ query: 'abc' }),
  },
})

Any change in the URLSearchParams will hit the same cache entry, which should not happen.

This happens because JSON.stringify (used here) serializes any URLSearchParams object to {}, because URLSearchParams does not implement a toJSON method. A solution could be to implement a replacer for JSON.stringify.

As a workaround, one can manually convert it to its string representation and set the Content-Type header (as undici's implementation would do automatically in the first example):

let params = new URLSearchParams({ query: 'abc' });

Fetch("https://example.com", {
  fetchOptions: {
    method: "POST",
    body: params.toString(),
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8',
    },
  },
})

Note: as mentioned in the MDN article, URLSearchParams is not the only kind of object one can pass as body, apart from a plain string. I only tested URLSearchParams.