getStoredPassword(username)
    .then(responseFromDB => {
      if (responseFromDB.length === 0)
        res.status(400).send({
          error: "User not found. Are you sure that's the right username?"
        });
...
...

      comparePasswords(password, hashed_password)
        .then(match => {
          if (match) {
            const jwt = createJWT(username);
            res.cookie("jwt", jwt, {
              httpOnly: true
            });
            res.status(200).end();
          } else {
            res.status(400).send({
              error:
                "Sorry, your password was incorrect. Please double-check your password."
            });
          }
        })
        .catch(passwordMatchErr => console.log);
    })
    .catch(databaseErr => console.log);

const [error,setError]=useState("")
if (response.status !== 200) {
      const responseJson = await response.json();
      setError(responseJson.error);
}