Why 'this' is undefined inside a Promise call

Question

I don't understand what's going on

componentDidMount() {
    console.log('componentDidMount');
    //const self = this; 
    let _id = this.props.match.params.id.toUpperCase();

    if (_id != this.state.id.toUpperCase()) {

        axios.get('/data/pricemultifull?fsyms=' + _id + '&tsyms=USD')
            .then(response => {
                // let _currentcoin = { ...resp.data.RAW.BTC.USD, ticker: _id };
                this.setState({ id: _id }); //this == undefined
            });
    }
}

I can get a response back but this is always undefined and I'm unable to setState. I'm using an arrow function which I thought was scope 'this' to the component level. I can fix it by making a new var and setting 'this' before I make the request. I know that this should be working though. What am I missing?

My entire component

import React, { Component } from 'react';
import axios from '../../axios';


class CoinViewer extends Component {

state = {
    coin: {},
    hasLoaded: false,
    id: ''
}

componentDidMount() {
    console.log('componentDidMount');
    //const self = this; 
    let _id = this.props.match.params.id.toUpperCase();

    if (_id != this.state.id.toUpperCase()) {

        axios.get('/data/pricemultifull?fsyms=' + _id + '&tsyms=USD')
            .then( resp => {
                // let _currentcoin = { ...resp.data.RAW.BTC.USD, ticker: _id };
                this.setState({ id: _id });
            });
    }
}

componentWillMount() {

}

componentWillUpdate() {


}

componentDidUpdate() {

}

getCompleteCoinData(_id) {

}


render() {


    return (
        <div>
            CoinViewer Component: {this.state.id} sads
        </div>
    )
}

}

export default CoinViewer

Answer 1

:Edit Wow, the below is kinda true, but the real issue is you didn't initialize state. https://reactjs.org/docs/react-component.html#constructor

constructor() {
  super();
  this.state = {
    coin: {},
    hasLoaded: false,
    id: ''
  }
}

You could use lexical scoping and fix like this, this is a popular pattern to protect this.

Basically, when you use promises or functions from other libraries/ APIs you do not know what they have set their context inside the callback functions to.

In order to use the context you want, you keep the context you need saved in a variable within scope and reference it there _this, rather than by pointing to the context this. I'd recommend reading 'you dont know js' to understand this concept further.

componentDidMount() {
  console.log('componentDidMount');
  const _this = this; 
  let _id = _this.props.match.params.id.toUpperCase();

  if ( _id != _this.state.id.toUpperCase() ) {
    axios.get('/data/pricemultifull?fsyms=' + _id + '&tsyms=USD')
      .then(response => {
        _this.setState({ id: _id }); //this == undefined
      });
  }
}

Answer 2

Solution 1: arrow functions..

requestSuccess = (resp) => {
  // let _currentcoin = { ...resp.data.RAW.BTC.USD, ticker: _id };
  this.setState({ id: _id });
}

componentDidMount() {
  console.log('componentDidMount');
  //const self = this; 
  let _id = this.props.match.params.id.toUpperCase();
  if (_id != this.state.id.toUpperCase()) {
     axios.get('/data/pricemultifull?fsyms=' + _id + '&tsyms=USD')
       .then(this.requestSuccess);
  }
}

Solution 2: binding

componentDidMount() {
  console.log('componentDidMount');
  //const self = this; 
  let _id = this.props.match.params.id.toUpperCase();
  if (_id != this.state.id.toUpperCase()) {
     axios.get('/data/pricemultifull?fsyms=' + _id + '&tsyms=USD')
       .then((resp) => {
          // let _currentcoin = { ...resp.data.RAW.BTC.USD, ticker: _id };
          this.setState({ id: _id });
     }.bind(this));
  }
}

Answer 3

When working with React.js, chances are you have faced a problem how to access this from inside the promise.There is more than one solution to resolve this reference inside the promise. The old approach would be setting the self = this reference While this would work, the recommended solution, which is more inline with ES6, would be to use an arrow function here:

class Component extends React.Component { 
  componentDidMount() {
    let component = this;
    axios.get('http://…').then(function(data) {
      component.setState( { name: data.blah } );
    });
  }
}

The arrow syntax, as stated above, is a much smarter way to allow use of this to make reference to React.Component classes, as we can see below:

class Component extends React.Component { 
  componentDidMount() {
    axios.get('http://…').then(data => {
      this.setState( { name: data.blah } );
    });
  }
}

Please note, instead of using function(data) { //body }, we used data => { //body }, and in this case this reference won’t get the promise instance back.