react.js:3640 Warning: Each child in an array or iterator should have a unique "key" prop. Check the...

react.js:3640 Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of UserList. See https://fb.me/react-warning-keys for more information.
in h3 (created by UserList)
in UserList

加上 key ， 如下：

render: function () {
            var createItem = function (u) {
                return (
 {u.username + ':' + JSON.stringify(u.roles)}
);
            }
            return (
                    

                        {this.state.users.map(createItem)}
                    
            );
        }