How can I validate a field using a JavaScript function?

Question

I created a form starting from the example in the docs in "Validation - Custom styles" section. I added "required" attr to the cumpolsory inputs and validation happens well. Now I would need to add custom valdation to a field that must respect a particular format.

I created the function validaForm() to validate the field and to add the "invalid" class input to it. I would need help in how to use the function in the code *** provided by the Bootstrap example:

// My custom function to validate the field
    function validaForm() {
      var $inputs = $("#frmIscrCant :input:not([readonly])");
      //var values = {};
      $inputs.each(function () {
        //values[this.id] = $(this).val();          
        if ($(this).hasClass("partIVAInput") && $(this).val().trim() != "")
          if (!isValidPIVA($(this).val())) {
            $(this).addClass("is-invalid");
            alert("P.IVA non valida!");
          }
      });
    }

// Code provided by Bootstrap example(***). In this function I need to insert the function above
(function() {
  'use strict';
  window.addEventListener('load', function() {
    // Fetch all the forms we want to apply custom Bootstrap validation styles to
    var forms = document.getElementsByClassName('needs-validation');
    // Loop over them and prevent submission
    var validation = Array.prototype.filter.call(forms, function(form) {
      form.addEventListener('submit', function(event) {
        validaForm(); // !!!
        if (form.checkValidity() === false) {
          event.preventDefault();
          event.stopPropagation();
        }
        form.classList.add('was-validated');
      }, false);
    });
  }, false);
})();
</script>