javascript - ReactJS 在渲染组件时表现得很奇怪

Question

在 ReactJS 中使用 map() 函数时，我遇到了一个奇怪的问题。我将 3 个对象的数组硬编码到我的 App 状态中，当我尝试遍历每个对象时，我可以在控制台中看到数据。看起来 TrackList.js 正在再次渲染，然后吐出一个未定义的错误。
这是我的主要文件:

App.js:

import React, { Component } from 'react';
import './App.css';
import SearchBar from '../SearchBar/SearchBar';
import SearchResults from '../SearchResults/SearchResults';
import Playlist from '../Playlist/Playlist';
// import songs from './songs.js';

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
      searchResults: [
        {
          "id": 1,
          "name": "Roses",
          "artist": "The Chainsmokers",
          "album": "Unknown",
          "isRemoval": false
        },
        {
          "id": 2,
          "name": "Fight Song",
          "artist": "Rachel Platten",
          "album": "None",
          "isRemoval": false
        },
        {
          "id": 3,
          "name": "Thunder",
          "artist": "Imagine Dragons",
          "album": "Evolve",
          "isRemoval": false
        }
      ]
    };
  }

  render() {
    return (
      <div>
        <h1>Ja<span className="highlight">mmm</span>ing</h1>
        <div className="App">
          <SearchBar />
          <div className="App-playlist">
            <SearchResults searchResults={this.state.searchResults}/>
            <Playlist />
          </div>
        </div>
      </div>
    );
  }
}

export default App;

搜索结果.js:

import React from 'react';
import './SearchResults.css';
import TrackList from '../TrackList/TrackList';

class SearchResults extends React.Component {
  render() {
    return (
      <div className="SearchResults">
        <h2>Results</h2>
        <TrackList tracks={this.props.searchResults} />
      </div>
    );
  }
}

export default SearchResults;

TrackList.js:

import React from 'react';
import './TrackList.css';
import Track from '../Track/Track';


class TrackList extends React.Component {
  render() {
    return (
      <div className="TrackList">
          {console.log("Tracks:")}
          {console.log(this.props.tracks)}
          {
            this.props.tracks.map(track => {
              return <Track track={track} key={track.id} />;
            })
          }
      </div>
    );
  }
}

export default TrackList;

Track.js:

import React from 'react';
import './Track.css';


class Track extends React.Component {
  constructor(props) {
    super(props);
    this.renderAction = this.renderAction.bind(this);
  }

  renderAction(isRemoval) {
    if (isRemoval) {
      return "-";
    }
    return "+";
  }

  render() {
    return (
      <div className="Track">
        <div className="Track-information">
        {console.log("Each track:")}
        {console.log(this.props.track)}
          <h3>{this.props.track.name}</h3>
          <p>{this.props.track.artist} | {this.props.track.album}</p>
        </div>
        <a className="Track-action">{this.renderAction(this.props.track.isRemoval)}</a>
      </div>
    );
  }
}

export default Track;

Console.log():

Tracks:
Array [ {…}, {…}, {…} ]
Each track:
Object { id: 1, name: "Roses", artist: "The Chainsmokers", album: "Unknown" }
Each track:
Object { id: 2, name: "Fight Song", artist: "Rachel Platten", album: "None" }
Each track:
Object { id: 3, name: "Thunder", artist: "Imagine Dragons", album: "Evolve" }
Tracks:
undefined
TypeError: this.props.tracks is undefined

Answer 1

尝试将 this.props.tracks.map(track => { 行替换为以下行，

this.props.tracks && this.props.tracks.map(track => {
    // code continues