[javascript] 반응 :“this”는 컴포넌트 함수 안에서 정의되지 않았다

class PlayerControls extends React.Component {
  constructor(props) {
    super(props)

    this.state = {
      loopActive: false,
      shuffleActive: false,
    }
  }

  render() {
    var shuffleClassName = this.state.toggleActive ? "player-control-icon active" : "player-control-icon"

    return (
      <div className="player-controls">
        <FontAwesome
          className="player-control-icon"
          name='refresh'
          onClick={this.onToggleLoop}
          spin={this.state.loopActive}
        />
        <FontAwesome
          className={shuffleClassName}
          name='random'
          onClick={this.onToggleShuffle}
        />
      </div>
    );
  }

  onToggleLoop(event) {
    // "this is undefined??" <--- here
    this.setState({loopActive: !this.state.loopActive})
    this.props.onToggleLoop()
  }

loopActive토글시 상태 를 업데이트하고 싶지만 this핸들러에서 객체가 정의되어 있지 않습니다. 튜토리얼 문서에 따르면 this구성 요소를 참조해야합니다. 뭔가 빠졌습니까?

답변

constructor (props){
  super(props);

  this.state = {
      loopActive: false,
      shuffleActive: false,
    };

  this.onToggleLoop = this.onToggleLoop.bind(this);

}

onToggleLoop = (event) => {
    this.setState({loopActive: !this.state.loopActive})
    this.props.onToggleLoop()
}

{someList.map(function(listItem) {
  // your code
}, this)}

{someList.map((listItem, index) =>
    <div onClick={this.someFunction.bind(this, listItem)} />
)}

onClick={this.onToggleLoop} // Here you just passing reference, React will invoke it as independent function and this will be undefined

onClick={()=>this.onToggleLoop()} // Here you invoking your desired function as method of this, and this in that function will be set to object from that function is called ie: your component object

export default class SignupPage extends React.Component {
  constructor(props) {
    super(props);
  }

  handleSubmit(e) {
    e.preventDefault();

    const data = {
      email: this.refs.email.value,
    }
  }

  render() {

    const {errors} = this.props;

    return (
      <div className="view-container registrations new">
        <main>
          <form id="sign_up_form" onSubmit={::this.handleSubmit}>
            <div className="field">
              <input ref="email" id="user_email" type="email" placeholder="Email"  />
            </div>
            <div className="field">
              <input ref="password" id="user_password" type="new-password" placeholder="Password"  />
            </div>
            <button type="submit">Sign up</button>
          </form>
        </main>
      </div>
    )
  }

}