formset.js

var FormSet = function (form, prefix) {
  this.$form = $(form);
  this.prefix = prefix;
}

FormSet.prototype.updateElementIndex = function (el, ndx) {
  var regex = new RegExp('(' + this.prefix + '-\\d+-)'),
      replacement = this.prefix + '-' + ndx + '-',
      element = $(el);

  if (element.attr('for')) {
    element.attr('for', element.attr('for').replace(regex, replacement));
  }

  if (element.attr('id')) {
    element.attr('id', element.attr('id').replace(regex, replacement));
  }

  if (element.attr('name')) {
    element.attr('name', element.attr('name').replace(regex, replacement));
  }
}

FormSet.prototype.count = function (count) {
  var key = '#id_' + this.prefix + '-TOTAL_FORMS',
      el = $(key);

  if (count === undefined) {
    return parseInt(el.val(), 10);
  }

  el.val(count);
}

FormSet.prototype.remove = function (btn, callback) {
  var formCount = this.count();

  if (formCount < 1) {
    return;
  }

  var parent = $(btn).parents('.item:first'),
      hidden = parent.find('input[type=checkbox][name*="DELETE"]'),
      forms = this.$form.find('.item'),
      i = 0,
      _this = this;


  if (hidden.length) {
    hidden.prop('checked', true);
    parent.hide();
  } else {
    parent.remove();

    this.count(forms.length);

    for (formCount = forms.length; i < formCount; i++) {
      $(forms.get(i)).children().children().each(function () {
        if ($(this).attr('type') == 'text') {
          _this.updateElementIndex(this, i);
        }
      });
    }
  }

  if (callback !== undefined) {
    callback(forms);
  }
}

FormSet.prototype.add = function (btn, callback) {
  var formCount = this.count(),
      last = this.$form.find('.item:last'),
      row = last.clone(false).get(0),
      _this = this;

  $(row).removeAttr('id').hide().insertAfter(last).slideDown(300);

  $('.errorlist', row).remove();

  $(row).find('input,textarea,select').each(function () {
    _this.updateElementIndex($(this), formCount);
    $(this).val('');
  });

  $(row).find('.delete').click(function () {
    return _this.remove($(this));
  });

  _this.count(formCount + 1);

  if (callback !== undefined) {
    callback(row);
  }
}