Easy Ui Jquery easyui-textbox change onChange event

$('#LastName').textbox({
    onChange: function (newValue, oldValue) {
        console.log('onchange fired...textbox, newValue:' + newValue + ";oldValue:" + oldValue);
        $("#Fullname").textbox({ value: $('#FirstName').val().trim() + " " + $('#MiddleName').val().trim() + " " + $('#LastName').val().trim() });
    }
});

<tr>
                                <td>
                                    <label style="font-size: 12px">First Name :</label>
                                </td>
                                <td>
                                    @*<input name="FirstName" id="FirstName" class="easyui-textbox" required="true"></input>*@
                                    @*<input name="FirstName"   class="easyui-textbox" required="true"></input>*@
                                    <input name="FirstName" id="FirstName" class="easyui-textbox" style="width:150px" ></input>
                                </td>

                                <td>
                                    <label style="font-size: 12px">Middle Name:</label>
                                </td>
                                <td>
                                    @*<input name="MiddleName" class="easyui-textbox" id="MiddleName"></input>*@
                                    @*<input name="MiddleName" class="easyui-textbox" ></input>*@
                                    <input name="MiddleName" id="MiddleName" class="easyui-textbox" style="width:150px"></input>
                                </td>
                            </tr>
                            <tr>
                                <td>
                                    <label style="font-size: 12px">Last Name:</label>
                                </td>
                                <td>
                                    @*<input name="LastName" class="easyui-textbox" id="LastName" required="true"></input>*@
                                    @*<input name="LastName" class="easyui-textbox"  required="true"></input>*@
                                    <input name="LastName" id="LastName" class="easyui-textbox" style="width:150px" required="true"></input>
                                </td>
                                
                                <td>
                                    <label style="font-size: 12px">Discount Percentage:</label>
                                </td>
                                <td>
                                    @*<input name="LastName" class="easyui-textbox" id="LastName" required="true"></input>*@
                                    @*<input name="LastName" class="easyui-textbox"  required="true"></input>*@
                                    <input name="DiscountPercentage" id="DiscountPercentage" class="easyui-textbox" value="0" style="width:150px" ></input>
                                </td>
                            </tr>
                            <tr>
                                <td>
                                    <label style="font-size: 12px">Full Name </label>
                                </td>
                                <td colspan="2" style="width: 300px">
                                    <input name="Fullname" id="Fullname" class="easyui-textbox" data-options="iconCls:'icon-man'" style="width:300px"></input>
                                </td>
                            </tr>