How to use fetch() API in React to setState

Question

I'm trying to write a component in React that will use the fetch() API to get data from a website, then use setState to set a state equal to the data, and then finally render the data. My code looks like this:

import React from 'react';

export default class Test extends React.Component {
    constructor(props){
        super(props);
        this.state = {apiInfo: 'default'};
    }

    componentDidMount(){
        fetch('https://fcctop100.herokuapp.com/api/fccusers/top/recent').then(
            function(response){
                return response.json();
            }
            ).then(function(jsonData){
                return JSON.stringify(jsonData);
            }
            ).then(function(jsonStr){
                this.setState({apiInfo: jsonStr});
                console.log(jsonStr);
            });
    }

    render(){
        return(
                <tr>
                    <td>{this.state.apiInfo}</td>
                </tr>
        );
    }
}

However, this results with an error saying I'm unable to setState of undefined. I end up rendering 'default' on my HTML. What exactly am I doing wrong here?

Answer 1

Your error message is telling you exactly what the problem is:

unable to setState of undefined

So you're trying call setState as a method of an object that doesn't exist at that point. As a property of what object are you trying to call setState as a method?

this.setState({apiInfo: jsonStr});

Yes, it's your this that's the problem. At the point that you're trying to call it - i.e. inside a .then() of a fetch call - this is actually undefined. You can see this in the Chrome Devtools:

I'm afraid that this is a slippery customer in JavaScript; its value can (and does) change depending upon the current context of your app.

There's several ways you can workaround this. One slightly clunky (but it works!) way is to capture your this value before you enter your .fetch() call, and assign it to another variable. You'll often see that or self variables used for this purpose, but they're just conventions. You can call the variable what you like.

Here's how I've reworked your componentDidMount() method capturing this to that, and calling that inside the .then():

componentDidMount() {
    const that = this;
    fetch("https://fcctop100.herokuapp.com/api/fccusers/top/recent")
        .then(function(response) {
            return response.json();
        })
        .then(function(jsonData) {
            return JSON.stringify(jsonData);
        })
        .then(function(jsonStr) {
            that.setState({ apiInfo: jsonStr });
            console.log(jsonStr);
        });
}

If you're comfortable using arrow functions, then another way is to replace your "normal" function call with one, like so:

.then(jsonStr => {
    this.setState({ apiInfo: jsonStr });
    console.log(jsonStr);
});

An arrow function's this is always the this that its parent defined.

Answer 2

It is saying setState is undefined because you're accessing it in the wrong context. You can either, convert the function into an arrow function or bind it to the right context. Here is an article as to When and why do we bind this to the React component method.

In your code, the change that can be made is either binding it

.then(function(jsonStr){
          this.setState({apiInfo: jsonStr});
          console.log(jsonStr);
      }.bind(this));

or using arrow function

.then((jsonStr)=>{
          this.setState({apiInfo: jsonStr});
          console.log(jsonStr);
      });