Failed to fetch Error - How do I send an axios response in Polka?

Question

I try to fetch login datas in a Svelte/Sapper app as in the code below and I use polka and axios for the api request on the server.

I don't understand the behaviour in the axios call. The console.log(JSON.stringify(result.data)) logs the correct result as I expected. But the fetch request on the client throws this exception:

Error: Failed to fetch

Can anyone explain me what I do wrong?

//client
try {
      const res = await fetch("login.json", {
        method: "POST",
        headers: {"Content-Type": "application/json"},
        body: JSON.stringify({ user: email, passw: password }),
      });
      let result = await res.json();
     
      if (!res) {
        throw new Error(result.error);
      }else{
        console.log("result: "+JSON.stringify(result));
      }
    } catch (e) {
      error = e.message;
      console.log("Error: "+error); //Error: Failed to fetch
    }
  }

//server
export async function post(req, res) {
    const password = sha3_512(req.body.password);
    const qs = require('qs');
    const data = qs.stringify({ user: req.body.user, password: password })
    const config = {
        method: 'post',
        url: 'https://****login',
        httpsAgent: agent,
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded'
        },
        data: data
    }
    
    axios(config)
    .then(result => {
        console.log(JSON.stringify(result.data));//successful response
        res.end(JSON.stringify(result.data));
    })
    .catch(err => res.end(err));
}

Answer 1

The problem is solved after add preventDefault in the form element.

 <form on:submit|preventDefault={() => submit()}>