code: function mirror(form_id, input_name) {
          var _this7 = this;
  
          this._checkInit(form_id);
          var input = this._collection[form_id].getInput(input_name);
          if (!(input instanceof HTMLInputElement)) {
              // make sure it is normal input and not RadioNodeList or other interfaces which don't have addEventListener
              throw new Error('Cannot mirror radio buttons or checkboxes.');
          }
          if (this._mirrorCollection[form_id] === undefined) {
              this._mirrorCollection[form_id] = {};
          }
          this._mirrorCollection[form_id][input_name] = true;
  
          //const input = document.forms[form_id].elements[input_name];
          this.setMirrors(form_id, input_name);
          input.addEventListener('change', function () {
              _this7.setMirrors(form_id, input_name);
          });
      }
docs: /*
  binding (mirror) methods
   
  Mirrors real DOM input's value with any DOM element (two-way data binding)
  All DOM elements with attribute data-mirror="form_id.input_name" are always updated when input value changed
  @param {string} form_id
  @param {string} input_name