Sveltekit : Unable to access a POST endpoint's reponse body as json

When trying to get a dummy response from a POST endpoint, the call to res.json() throws a serialization error in the client :

Uncaught (in promise) SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

This is the client side :

const body = /* request body */
const res = await fetch(/* url */, {
  method: 'POST',
  body: JSON.stringify(body),
})
console.log(await res.json())

And this is the endpoint :

export const POST = async ({request}) => {
  /* do something with the request's body */
  return {
    body: {a: 1}
  }
}

I get the same error on the server side if I don’t stringify the body in fetch, but I can’t stringify it in the endpoint, as only plain objets (and errors) are allowed.

The outcome is the same with an empty object.

>Solution :

You have to instruct the endpoint to send JSON, otherwise it will send the associated page as server-side rendered HTML. To do that add an Accept header:

const res = await fetch(/* url */, {
  method: 'POST',
  headers: {
    'Accept': 'application/json'
  },
  body: JSON.stringify(body),
})