Error message when server returns undefined error. If you do not like default message, you are welcome to define your own. let form = new w2form({ name : 'form', header : 'Form Header', msgAJAXerror : 'Uncaught AJAX error', fields: [ { field: 'first_name', type: 'text', required: true }, { field: 'last_name', type: 'text', required: true }, { field: 'email', type: 'email' } ], record: { first_name : 'John', last_name : 'Doe', email : 'jdoe@email.com' }, actions: { "save": function () { this.validate(); }, "reset": function () { this.clear(); } } });