jquery.form.js

;(function($) {
  $.fn.extend({
    ajaxForm: function(){
      var action = this.attr('action');
      var method = this.attr('method');
      var d = $.Deferred();
      this.on('submit', function(){
        fetch(action, {
          method: method,
          credentials: 'same-origin',
          body: new FormData(this[0]),
        }).then(function(data){
          d.resolve(data);
        }).catch(function(data){
          d.reject(data);
        });
        return false;
      });
      return d.promise();
    },
  });
})(jQuery);

$('form').ajaxForm()
.then(function(res){
  return res.json();
}).then(function(json){
  // handle json
});