无法读取ReactJS（盖茨比）中未定义的属性“状态”[重复]

Question

我有以下代码：

export default class Contact extends React.Component {
    constructor(props) {
      super(props);
      this.state = {
        password: '',
        redirect: false,
        username: ''
      };

  this.onUsernameChange = this.onUsernameChange.bind(this);
  this.onPasswordChange = this.onPasswordChange.bind(this);
}

onUsernameChange(event) {
  this.setState({ username: event.target.value });
}

onPasswordChange(event) {
  this.setState({ password: event.target.value });
}

handleSubmit(event) { 
  event.preventDefault();
  alert('A name was submitted: ' + this.state.username);

  //sessionStorage.setItem('username', this.state.username);

}

render() {

  return (
      <div>
        <form className="form-signin" onSubmit={this.handleSubmit}>
          <input type="text" value={this.username} onChange={this.onUsernameChange} />
          <input type="password" value={this.password} onChange={this.onPasswordChange} />
          <input type="submit" value="Submit" />
        </form>
      </div>
  );
}
}

提交用户名和密码后：出现以下错误：

TypeError: Cannot read property 'state' of undefined

在以下行：

alert('A name was submitted: ' + this.state.username);

有什么问题？用户名不保存吗？（我用的是盖茨比）

Answer 1

我需要补充：

this.handleSubmit = this.handleSubmit.bind(this); 

在构造函数中！

Answer 2

您还可以对函数使用es6格式，这将防止每次在构造函数中为绑定此上下文编写额外代码。例如

onUsernameChange = event => {
  this.setState({ username: event.target.value });
}

Answer 3

您还忘了绑定handleSubmit

做：

this.onUsernameChange = this.onUsernameChange.bind(this);
this.onPasswordChange = this.onPasswordChange.bind(this);
this.handleSubmit = this.handleSubmit.bind(this);

在构造函数中