Custom form validation with jQuery

Im using jQuery to make a custom form validation.

My issue is that when its alerting an error, i get every error in a separate alert dialog.. my goal is to have it list all errors in 1 single alert.

How do i achieve that?

This is my code:

jQuery(document).ready(function() {

  jQuery("form.kontakt-form").on("submit", function(e){
    jQuery(".kontakt-form input").each(function(){

      var name = jQuery(this).attr("placeholder");

      if(jQuery(this).val() == ""){
        alert("There was an error" + " " + name);
        e.preventDefault();
      }else{
        return true; 
      }
    });
  });

})