Checking Form Fields With Ajax And Php

I am trying to write a code that will check in a php file if username/mail has already been registered. The following code doesn't work because in check_fields() when check_field()

Solution 1:

Chain the AJAX calls for each field. The callback for field i submits the AJAX call to check field i+1, and so on. The last callback submits the form if all were successful.

functioncheck_fields(form, items) {
    var success = true;
    functioncheck_field(i) {
        var id = items[i];
        var value = document.getElementById(id).value;
        ajax_php(id, value, 'check_field.php', function(returned_value) {
            document.getElementById(id + '_err').textContent = returned_value;
            if (returned_value != '') {
                success = false;
            }
            if (++i < items.length) {
                check_field(i); // Check the next field
            } elseif (success) { // reached the last field with no errors
                form.submit();
            }
        });
    }
    check_field(0);
}

functioncheck(form) {
    check_fields(form, ['username', 'mail']);
    returnfalse; // prevent form submission
}

The form should contain:

onsubmit="return check(this)"