render() {
            const users = this.state.words.map( key => <li> {key} </li>)
        return (
            <div className="todo-item">
                {users}
            </div> 
        )Objects are not valid as a React child (found: object with keys {key}). If you meant to render a collection of children, use an array instead.
При следующих вопросах обязательно учту ваши замечания!