login.js

var isLogin;
  var submitButton;

  document.forms['customer_login'].addEventListener('submit', (event) => {
 
    // show loading gif on submit button
    submitButton = event.target.querySelector('button');
    submitButton.classList.add('spin');
    submitButton.disabled = true;

    // prevent form submit
    event.preventDefault();
 
    // boolean flag - form is for login/register
    isFormLogin = event.target.action.includes('account/login')?true:false;

    // fetch call to get answer from shopify
    fetch(event.target.action, {
        method: 'POST',
        body: new URLSearchParams(new FormData(event.target))
    })
    .then((response) => {
 
      // boolean flag - redicrect url is login/account (login/register success)
      isLogin = response.url.includes('account/login');

      // return response THML body
      return response.text(); 
 
    })
    .then(body => {
      
      // hide loading gif on submit button
      submitButton.classList.remove('spin');
      submitButton.disabled = false;
    
      if(isFormLogin){
 
        if(!isLogin){

          // go to redirect
          window.location.href = 'account';
        }
        else{

          // show error on page
          document.querySelector('#formErrorsSpan').innerHTML = 'Login Failed';
        }
      }
      else{

        // create elemnt to parse the response body to HTML and get the DOM  element holding the errors
        var el = document.createElement( 'html' );
        el.innerHTML = body;
        let errors = el.getElementsByClassName("errors")[0];
        if(errors){

          // if exist - map to string array
          errors = Array.from(el.getElementsByClassName("errors")[0].getElementsByTagName("UL")[0].getElementsByTagName("li")).map(li=>{
            return li.innerHTML;
          }); 
          
          // show errors on page - with line-break between them if multiple
          document.querySelector('#formErrorsSpan').innerHTML = errors.join('<br>');
        }
        else{
        
          // go to redirect
          window.location.href = 'account';
       }
      }
    })
    .catch((error) => {
      console.log('error: ' + error);
      // TODO handle error
    });
});