与ES7反应：未捕获TypeError：无法读取未定义

Question

我收到此错误 未捕获的TypeError：每当我在AuthorForm的输入框中输入任何内容时，都无法读取未定义的属性“状态” 。我正在使用React和ES7。

错误发生在ManageAuthorPage 中 setAuthorState函数的第3行。即使我在setAuthorState中放置了console.log（this.state.author），它也会停在console.log并调出错误。

无法通过互联网找到其他人的类似问题。

以下是 ManageAuthorPage 代码：

import React, { Component } from 'react';
import AuthorForm from './authorForm';

class ManageAuthorPage extends Component {
  state = {
    author: { id: '', firstName: '', lastName: '' }
  };

  setAuthorState(event) {
    let field = event.target.name;
    let value = event.target.value;
    this.state.author[field] = value;
    return this.setState({author: this.state.author});
  };

  render() {
    return (
      <AuthorForm
        author={this.state.author}
        onChange={this.setAuthorState}
      />
    );
  }
}

export default ManageAuthorPage 

这是 AuthorForm 代码：

import React, { Component } from 'react';

class AuthorForm extends Component {
  render() {
    return (
      <form>
                <h1>Manage Author</h1>
        <label htmlFor="firstName">First Name</label>
                <input type="text"
                    name="firstName"
          className="form-control"
                    placeholder="First Name"
          ref="firstName"
          onChange={this.props.onChange}
                    value={this.props.author.firstName}
          />
        <br />

        <label htmlFor="lastName">Last Name</label>
                <input type="text"
                    name="lastName"
          className="form-control"
                    placeholder="Last Name"
          ref="lastName"
                    onChange={this.props.onChange}
          value={this.props.author.lastName}
                    />

                <input type="submit" value="Save" className="btn btn-default" />
            </form>
    );
  }
}

export default AuthorForm

Answer 1

您必须将事件处理程序绑定到正确的上下文（this）：

onChange={this.setAuthorState.bind(this)}

Answer 2

确保您在构造函数中首先调用super()！

您应为this方法

设置setAuthorState

class ManageAuthorPage extends Component {

  state = {
    author: { id: '', firstName: '', lastName: '' }
  };

  constructor(props) {
    super(props);
    this.handleAuthorChange = this.handleAuthorChange.bind(this);
  } 

  handleAuthorChange(event) {
    let {name: fieldName, value} = event.target;

    this.setState({
      [fieldName]: value
    });
  };

  render() {
    return (
      <AuthorForm
        author={this.state.author}
        onChange={this.handleAuthorChange}
      />
    );
  }
}

基于arrow function的另一种选择：

class ManageAuthorPage extends Component {

  state = {
    author: { id: '', firstName: '', lastName: '' }
  };

  handleAuthorChange = (event) => {
    const {name: fieldName, value} = event.target;

    this.setState({
      [fieldName]: value
    });
  };

  render() {
    return (
      <AuthorForm
        author={this.state.author}
        onChange={this.handleAuthorChange}
      />
    );
  }
}