Called when server returns an error. This event is triggered each time an error message needs to be displayed. In most cases the error message is displayed when server returns an error, but you can call .error() method on your own errors and it will trigger error event. You can add event listener during the object creation: let form = new w2form({ name : 'form', url : 'server/post', record: { field_1 : 1, field_2 : 2 }, fields: [ { field: 'field_1', type: 'text', required: true }, { field: 'field_2', type: 'alphanumeric', required: true } ], onError: function(event) { console.log(event); } }); or after the object has been created: w2ui.form.on('error', function(event) { console.log(event); }); The event handler is called before the default action of the event is triggered. You can cancel the default action by calling event.preventDefault(). To perform an action after the event is fully processed, define event.onComplete function. See events page in utilities for more details.