React - 访问返回数据中的嵌套对象

Question

我正在使用fetch 从本地 API 返回一些数据。我能够返回数据并设置状态。但是，在我的render() 函数中，当使用map 并尝试访问比数据中的顶级对象更深的任何内容时，我收到undefined 错误。我可以正确地将我想要的任何级别的数据记录到控制台，但我无法在呈现的组件中访问它。

constructor(props) {
  super(props);
  this.state ={
    verifications: []
  }
}

componentWillMount(){
  fetch('http://localhost:3001/verifications')
    .then(response => response.json())
    .then((verifications) => {
      this.setState({verifications})
      console.log(this.state);
    });
}

在我的渲染中

{this.state.verifications.map(verification =>
  <div key={verification._id}>
    <ReviewListItem
      hasAvatar={true}
      imageUrl={verification.supplier.logo}
      title={verification.supplier.companyName}
      description={verification.tasks.length}
    />
  </div>
)}

这是我得到的错误： Unhandled Rejection (TypeError): Cannot read property 'logo' of null

我已经到处寻找这个问题的答案，但我觉得我一定是在接近这个错误。我来自 Angular 1，我对 React 非常陌生。也许我很难掌握这些概念。

这是我的数据：

[
{
"_id": 1000,
"supplier": {
"_id": 1000,
"companyName": "ACME Business Ventures",
"logo": "/images/logos/suppliers/acme.jpg",
"avettaId": "ADS83J",
"brandColor": "#563E5E",
"clients": [],
"locations": [],
"primaryContact": {},
"address": {},
"createdAt": "2017-06-30T17:42:23.479Z"
},
"tasks": [
{
"form": "PQF General",
"question": "How long have you been in business?",
"answer": "18 Years",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "PQF General",
"question": "Have you had any fatalities in the last 3 years?",
"answer": "No",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Diversity Questionnaire",
"question": "Are you a minority-owned business?",
"answer": "Yes",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Sustainability Questionnaire",
"question": "What percentage of your business runs on renewable energy?",
"answer": "About 55%, but that's expected to climb to 80% by next year.",
"status": "incomplete",
"attachment": "",
"comments": []
}
]
},
{
"_id": 2000,
"supplier": null,
"tasks": [
{
"form": "PQF General",
"question": "How long have you been in business?",
"answer": "18 Years",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "PQF General",
"question": "Have you had any fatalities in the last 3 years?",
"answer": "No",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Diversity Questionnaire",
"question": "Are you a minority-owned business?",
"answer": "Yes",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Sustainability Questionnaire",
"question": "What percentage of your business runs on renewable energy?",
"answer": "About 55%, but that's expected to climb to 80% by next year.",
"status": "incomplete",
"attachment": "",
"comments": []
}
]
},
{
"_id": 3000,
"supplier": null,
"tasks": [
{
"form": "PQF General",
"question": "How long have you been in business?",
"answer": "18 Years",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "PQF General",
"question": "Have you had any fatalities in the last 3 years?",
"answer": "No",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Diversity Questionnaire",
"question": "Are you a minority-owned business?",
"answer": "Yes",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Sustainability Questionnaire",
"question": "What percentage of your business runs on renewable energy?",
"answer": "About 55%, but that's expected to climb to 80% by next year.",
"status": "incomplete",
"attachment": "",
"comments": []
}
]
},
{
"_id": 4000,
"supplier": null,
"tasks": [
{
"form": "PQF General",
"question": "How long have you been in business?",
"answer": "18 Years",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "PQF General",
"question": "Have you had any fatalities in the last 3 years?",
"answer": "No",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Diversity Questionnaire",
"question": "Are you a minority-owned business?",
"answer": "Yes",
"status": "incomplete",
"attachment": "",
"comments": []
},
{
"form": "Sustainability Questionnaire",
"question": "What percentage of your business runs on renewable energy?",
"answer": "About 55%, but that's expected to climb to 80% by next year.",
"status": "incomplete",
"attachment": "",
"comments": []
}
]
}
]

Answer 1

您的一个供应商为空（至少一个）。考虑使用 imageUrl={verification.supplier ? verification.supplier.logo : null} 之类的东西，而不是 imageUrl={verification.supplier.logo}。

Answer 2

数组的第二个索引是supplier: null。您可以先检查供应商是否存在

{this.state.verifications.map(verification =>
  <div key={verification._id}>
    <ReviewListItem
      hasAvatar={true}
      imageUrl={verification.supplier && verification.supplier.logo}
      title={verification.supplier && verification.supplier.companyName}
      description={verification.tasks.length}
    />
  </div>
)}

Answer 3

供应商在您的 json sn-p 中显然为空：

"supplier": null,

因此它显然失败了:)

我相信 Angular 会自动检查如下表达式中的空值：foo.bar.baz.something.deeply.nested——因此，如果任何内容为空，则结果将为空——在反应中（以及一般的 js 中），你需要自己检查空值。