reactjs - React this is undefined

Question

React this is undefined in promise . Here is my code:

export default class Album extends React.Component {

    constructor(props) {
        super(props);
    }

    componentDidMount ()  {
        console.log(this.props.route.appState.tracks);  // `this` is working
        axios({
            method: 'get',
            url: '/api/album/' + this.props.params.id + '/' + 'tracks/',
            headers: {
                'Authorization': 'JWT ' + sessionStorage.getItem('token')
            }
        }).then(function (response) {
            console.log(response.data);
            this.props.route.appState.tracks.concat(response.data); // 'this' isn't working
        }).catch(function (response) {
            console.error(response);
            //sweetAlert("Oops!", response.data, "error");
        })
    }

here is the error code:

TypeError: this is undefined
Stack trace:
componentDidMount/<@webpack:///./static/apps/components/Album.jsx?:81:17

Answer 1

Probably it's not binding this.

Try replacing with arrow functions if you're able to use ES6 syntax. It automatically binds this:

.then( (response) => {
        console.log(response.data);
        this.props.route.appState.tracks.concat(response.data); // 'this' isn't working
    } )

Or bind manually:

.then(function (response) {
            console.log(response.data);
            this.props.route.appState.tracks.concat(response.data);
        }.bind(this) )