If the entry conditions are insufficient on the membership registration page, we want to prevent submission after displaying a warning window, but it is not possible.

// Basic form validation implementation. Haven't been tested
function submitAction(event) {

  // Specifies the validation discrimination variables.
  var valid = true;
  var errors = [];

  // Define the rule.
  // When will it become invalid, what error messages will you show then, etc
  var a = $('input[name=username]').val();
  if(a.length===0){
    valid = false;
    errors.push ("Enter your ID";
  }

  // If you didn't pass the rules,
  if (!valid) {

    // I'll leave a message
    $(errors).each(function(error){
      alert(error);
    });

    // Prevents default behavior (submit form).
    return false;

  // If you've passed the rule, just tell them to do the basic action.
  } } else {
    return true;
  }
}

html
    head
        script(src="//code.jquery.com/jquery-3.3.1.min.js")
        title='Membership page'

        script.
            function submitAction() {
                var a = $('input[name=username]').val();
                if(a.length==0){
                    alert('Enter ID');
                    $('#frm').attr('method', 'get');
                    $('#frm').attr('action', 'register_');      
                }
            }


    body
        form(action='register', method='post', id='frm', onsubmit='submitAction();')

            input (type='text', name='username', placeholder='ID')

            p

            input (type='password', name='password', placeholder='password')

            p

            input (type='submit', value='membership')

app.get('/register_', function(req, res){
    res.redirect('/register');
})

onsubmit='submitAction' <= parentheses must be subtracted.

function submitAction(event) {
    event.preventDefault();
    // // blah blah...
}