在React中解析JSON输入时出错

Question

I'm writing a React App that gets JSON data from an API and shows some of it's contents.

This is the data it gets:

{"id":6,"name":"5 Storey I=0.7","Value":1344981250,"NSt":5,"jRatio":"[0.2,0.4,0.4]","jEDR":"[0.02,0.1,0.5,1]"}

And this is the App:

class App extends React.Component{
    constructor(props){
        super(props);
        this.state={
            data: [],
            isLoading: false,
            error: null,
        };
    }
    componentDidMount(){
        this.setState({isLoading: true});
        axios.get(API)
            .then(response => console.log(response.data.Value))
            .then(response => this.setState({data: response.data, isLoading: false}))
            .catch(response => this.setState({error: response.error, isLoading: false}));
    }
    render(){
        return (
            <div>
            <p>{this.state.error}</p>
            <p>{this.state.isLoading ? 'Loading...':'Loaded'}</p>
            <ul>{this.state.data.Value.toString()}</ul>
            </div>
        )
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

I get the Value "1344981250" in console but the page throws this error:

TypeError: this.state.data.Value is undefined

and in console:

The development server has disconnected. Refresh the page if necessary.

I also tried this:

this.state.data.map(obj => <li key={obj.toString()}>{obj.toString()}</li>)

and this time without any error, nothing shows up on the page. (this one works for arrays of objects but shows nothing when it's just one object)

So what did I miss?

Answer 1

TypeError: this.state.data.Value is undefined

Because data is an array and [].Value (any key) will be undefined.

Check this snippet:

 let data = []; console.log('data.Value = ', data.Value); // this will throw error: console.log('data.value.toString', data.Value.toString()) 

---

You defined the initial value of data as an array, and you are getting the object from api call. Solution is define the initial value as:

data: {}

And inside render method, write it like this:

<ul>{(this.state.data.Value || '').toString()}</ul>

You need to return the response from .then :

axios.get(API)
  .then(response => {console.log(response.data.Value); return response;}   //<=== here
  .then(response => this.setState({data: response.data, isLoading: false}))
  .catch(response => this.setState({error: response.error, isLoading: false}))

Edit:

componentDidMount will get called after the initial rendering, so its better to define the initial value of isLoading: true , after that you can remove this:

this.setState({isLoading: true});