Using Fetch API to create a Login form

So, I am making a login form and I have already implemented the fetch API

const form = {
    email: document.querySelector("#signin-email"),
    password: document.querySelector("#signin-password"),
    submit: document.querySelector("#signin-btn-submit"),
    messages:document.getElementById("form-messages")
};
let button = form.submit.addEventListener("click", (e)=> {
    e.preventDefault();
    const login = 'https://ffcc-app.herokuapp.com/user/login';

    fetch(login, {
          method: "POST",
          headers: {
            Accept: "application/json, text/plain, */*",
            "Content-Type": "application/json",
          },
          body: JSON.stringify({
            email: form.email.value,
            password: form.password.value,
          }),
        })
          .then((response) => response.json())
          .then((data) => console.log(data))
          .catch((err) => {
            console.log(err);
           });
    
    
});

I am also getting the desired output: This is my output

The problem is I have to redirect to the main page when 'login successful' else I have to display a message saying 'user does not exist'. how should I do that? What else should I add to my code?

Solution

const form = {
  email: document.querySelector("#signin-email"),
  password: document.querySelector("#signin-password"),
  submit: document.querySelector("#signin-btn-submit"),
  messages: document.getElementById("form-messages"),
};
let button = form.submit.addEventListener("click", (e) => {
  e.preventDefault();
  const login = "https://ffcc-app.herokuapp.com/user/login";

  fetch(login, {
    method: "POST",
    headers: {
      Accept: "application/json, text/plain, */*",
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      email: form.email.value,
      password: form.password.value,
    }),
  })
    .then((response) => response.json())
    .then((data) => {
      console.log(data);
      // code here //
      if (data.error) {
        alert("Error Password or Username"); /*displays error message*/
      } else {
        window.open(
          "target.html"
        ); /*opens the target page while Id & password matches*/
      }
    })
    .catch((err) => {
      console.log(err);
    });
});