为什么我无法在 React 中访问某些嵌套的 JSON 对象？

Question

我试图通过构建一个简单的获取和渲染应用来练习我的 React 技能。 我正在使用 Scryfall api 来搜索 MTG 卡。 我能够渲染卡片名称和艺术家等属性，但无法渲染价格和图像等属性。 这是我的 App.js：

import React from "react";
import scryfall from "../api/Scryfall";
import SearchBar from "./SearchBar";
import CardInfo from "./CardInfo";

class App extends React.Component {
  state = { card: {} };

  onSearchSubmit = async (name) => {
    const response = await scryfall.get("/cards/named", {
      params: { fuzzy: name },
    });

    this.setState({ card: response.data });
    console.log(response.data);
    console.log(response.data.prices.usd);
    console.log(response.data.image_uris.normal);
  };

  render() {
    return (
      <div className="ui container" style={{ marginTop: "10px" }}>
        <SearchBar onSubmit={this.onSearchSubmit} />
        <div>Found: {this.state.card.name}</div>
        <div>Artist: {this.state.card.artist}</div>
        <div>Price: {this.state.card.prices.usd}</div>
         <img
          src={this.state.card.image_uris.normal}
          alt={this.state.card.name}
        /> 


        <CardInfo card={this.state.card} />
      </div>
    );
  }
}

export default App;

当我 console.log response.data.prices.usd 或 respsonse.data.image_uris.normal 时，价格和图像 url 分别打印。 但是，当我 go 渲染它们时，我收到一条错误消息，提示“无法读取未定义的“usd”/“normal””。 我错过了什么？

这是 API 的信息：

Content-Type: application/json; charset=utf-8

{
  "object": "card",
  "id": "bef16a71-5ed2-4f30-a844-c02a0754f679",
  "oracle_id": "09cc8709-fe10-472a-b05c-e89f3523018d",
  "multiverse_ids": [
    438576
  ],
  "mtgo_id": 65899,
  "mtgo_foil_id": 65900,
  "tcgplayer_id": 145297,
  "name": "Austere Command",
  "lang": "en",
  "released_at": "2017-11-17",
  "uri": "https://api.scryfall.com/cards/bef16a71-5ed2-4f30-a844-c02a0754f679",
  "scryfall_uri": "https://scryfall.com/card/ima/10/austere-command?utm_source=api",
  "layout": "normal",
  "highres_image": true,
  "image_uris": {
    "small": "https://img.scryfall.com/cards/small/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.jpg?1562853529",
    "normal": "https://img.scryfall.com/cards/normal/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.jpg?1562853529",
    "large": "https://img.scryfall.com/cards/large/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.jpg?1562853529",
    "png": "https://img.scryfall.com/cards/png/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.png?1562853529",
    "art_crop": "https://img.scryfall.com/cards/art_crop/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.jpg?1562853529",
    "border_crop": "https://img.scryfall.com/cards/border_crop/front/b/e/bef16a71-5ed2-4f30-a844-c02a0754f679.jpg?1562853529"
  },
  "mana_cost": "{4}{W}{W}",
  "cmc": 6,
  "type_line": "Sorcery",
  "oracle_text": "Choose two —\n• Destroy all artifacts.\n• Destroy all enchantments.\n• Destroy all creatures with converted mana cost 3 or less.\n• Destroy all creatures with converted mana cost 4 or greater.",
  "colors": [
    "W"
  ],
  "color_identity": [
    "W"
  ],
  "legalities": {
    "standard": "not_legal",
    "future": "not_legal",
    "historic": "not_legal",
    "pioneer": "not_legal",
    "modern": "legal",
    "legacy": "legal",
    "pauper": "not_legal",
    "vintage": "legal",
    "penny": "not_legal",
    "commander": "legal",
    "brawl": "not_legal",
    "duel": "legal",
    "oldschool": "not_legal"
  },
  "games": [
    "paper",
    "mtgo"
  ],
  "reserved": false,
  "foil": true,
  "nonfoil": true,
  "oversized": false,
  "promo": false,
  "reprint": true,
  "variation": false,
  "set": "ima",
  "set_name": "Iconic Masters",
  "set_type": "masters",
  "set_uri": "https://api.scryfall.com/sets/741bcd30-7709-4133-8919-f4b46483bed7",
  "set_search_uri": "https://api.scryfall.com/cards/search?order=set&q=e%3Aima&unique=prints",
  "scryfall_set_uri": "https://scryfall.com/sets/ima?utm_source=api",
  "rulings_uri": "https://api.scryfall.com/cards/bef16a71-5ed2-4f30-a844-c02a0754f679/rulings",
  "prints_search_uri": "https://api.scryfall.com/cards/search?order=released&q=oracleid%3A09cc8709-fe10-472a-b05c-e89f3523018d&unique=prints",
  "collector_number": "10",
  "digital": false,
  "rarity": "rare",
  "card_back_id": "0aeebaf5-8c7d-4636-9e82-8c27447861f7",
  "artist": "Anna Steinbauer",
  "artist_ids": [
    "3516496c-c279-4b56-8239-720683d03ae0"
  ],
  "illustration_id": "7c6a01f8-e1f6-4fe4-b275-b2582be98783",
  "border_color": "black",
  "frame": "2015",
  "full_art": false,
  "textless": false,
  "booster": true,
  "story_spotlight": false,
  "edhrec_rank": 191,
  "prices": {
    "usd": "8.23",
    "usd_foil": "11.01",
    "eur": "6.04",
    "tix": "0.94"
  },
  "related_uris": {
    "gatherer": "https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=438576",
    "tcgplayer_decks": "https://decks.tcgplayer.com/magic/deck/search?contains=Austere+Command&page=1&partner=Scryfall&utm_campaign=affiliate&utm_medium=api&utm_source=scryfall",
    "edhrec": "https://edhrec.com/route/?cc=Austere+Command",
    "mtgtop8": "https://mtgtop8.com/search?MD_check=1&SB_check=1&cards=Austere+Command"
  },
  "purchase_uris": {
    "tcgplayer": "https://shop.tcgplayer.com/product/productsearch?id=145297&partner=Scryfall&utm_campaign=affiliate&utm_medium=api&utm_source=scryfall",
    "cardmarket": "https://www.cardmarket.com/en/Magic/Products/Singles/Iconic-Masters/Austere-Command?referrer=scryfall&utm_campaign=card_prices&utm_medium=text&utm_source=scryfall",
    "cardhoarder": "https://www.cardhoarder.com/cards/65899?affiliate_id=scryfall&ref=card-profile&utm_campaign=affiliate&utm_medium=card&utm_source=scryfall"
  }
}

Answer 1

最初你的 React 组件 state 有state = { card: {} };

但是您正在尝试从this.state.card中读取image_uris和name ，但它们不存在。

        <div>Price: {this.state.card.prices.usd}</div>
         <img
          src={this.state.card.image_uris.normal}
          alt={this.state.card.name}
        /> 

如果这些值未定义，您可能想放置一个 if 语句而不渲染组件。

例如在下面的代码中，我在渲染组件之前检查this.state.card.name是否存在。

  render() {

    if (!this.state.card.name) return null; 

    return (
      <div className="ui container" style={{ marginTop: "10px" }}>
        <SearchBar onSubmit={this.onSearchSubmit} />
        <div>Found: {this.state.card.name}</div>
        <div>Artist: {this.state.card.artist}</div>
        <div>Price: {this.state.card.prices.usd}</div>
         <img
          src={this.state.card.image_uris.normal}
          alt={this.state.card.name}
        /> 


        <CardInfo card={this.state.card} />
      </div>
    );
  }