如何使用我使用 fetch API 调用检索到的数据更新 Reactjs 状态？

Question

I made a fetch API call in react.js and put it inside a variable defined within the function containing the fetch function. But how do I transfer this value to one of the variables in the state? I can get to the point where I console.log the variable, but still I'm not able to figure out how to update one of the state variables so that I can then display the retrieved data onto the page.

import React from 'react';

class Stock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      stockInfo: '100'
    }
  }

  componentDidMount() {
    this.fetchStock();
  }

  fetchStock() {
    const API_KEY = 'api key goes here';
    let TimeInterval = '60min';
    let StockSymbol = 'AMZN';
    let API_Call = `https://www.alphavantage.co/query?function=TIME_SERIES_INTRADAY&symbol=${StockSymbol}&interval=${TimeInterval}&outputsize=compact&apikey=${API_KEY}`;
    let stockHistoryDatabase = {};
    let stockHistoryDatabaseString;

    fetch(API_Call)
      .then(
        function(response) {
          return response.json();
        }
      )
      .then(
        function(data) {
          console.log(data);

          for (var key in data['Time Series (60min)']) {
            // push the key value pair of the time stamp key and the opening value key paired together into an object with a key value pair data set storage.
            var epochKeyTime = new Date(key);
            epochKeyTime = epochKeyTime.getTime();
            stockHistoryDatabase[epochKeyTime] = data['Time Series (60min)'][key]['1. open'];
          }

          console.log(stockHistoryDatabase);
          stockHistoryDatabaseString = JSON.stringify(stockHistoryDatabase);
          console.log(stockHistoryDatabaseString);
        }
      );
  }

  handleChange = () => {
    this.setState({
      stockInfo: 'hello'
    });
  }

  render() {
    return(
      <div>
        <h1>Stocks</h1>
        <p>{this.state.stockInfo}</p>
        <button onClick={this.handleChange}>Change</button>
      </div>
    );
  }
}

export default Stock;

this is my code in entirety. I know how to change the state using a separate function that is called from a button click on the same page, but I'm unable to get the data stored in the variable 'stockHistoryDatabaseString' to replace the state 'stockInfo'.

Thank you for the help!

Answer 1

Since you are calling fetchStock after component is mounting. You can use arrow function as follows.

.then((data) => {
   // use data here
   this.setState({ ... }) // set you state
})

or if you are not comfortable using arrow function, then I believe you can create a function to handle the promise eg handleData

.then(this.handleData)

in class

// pseudo code

class YourClass extends React.Component {
  componentDidMount() {
    this.fetchStock()
  }
  handleData = (data) => {
    // process your data and set state
  }
  fetchStock() {
    // your API call
    fetch(API_CALL).then(this.handleData);
  }
  render() {}
}

If you are invoking fetchStock on user operation, such as button click, then you can provide appropriate context to fetchStock by binding it to React class you've created as follows:

constructor() {
  this.fetchStock = this.fetchStock.bind(this);
}

or there is another way to achieve the same (perhaps cleaner way):

fetchStock = () => {

}

Answer 2

First inside constructor add

this.fetchStock = this.fetchStock.bind(this);

Answer 3

I had a similar problem. My solution to this problem was to store this context of react class into one variable and then use it in any scope below it.

fetchStock() {
 const pointerToThis = this; // points to context of current react class
 fetch(API_Call)
  .then(function(response) {
    return response.json();
  })
  .then(function(data) {
    console.log(pointerToThis); // you can use pointerToThis which in turn points to react class 
  });
}