React - uncaught TypeError：無法讀取undefined的屬性'setState'我收到以下錯誤未捕獲的TypeError：無法讀取未定義的屬性“setState”甚至在構造函數中綁定delta之后。class Counter extends React.Component {     constructor(props) {         super(props);         this.state = {             count : 1         };         this.delta.bind(this);     }     delta() {         this.setState({             count : this.state.count++         });     }     render() {         return (             <div>                 <h1>{this.state.count}</h1>                 <button onClick={this.delta}>+</button>             </div>         );     }}