Using state in react with TypeScript

I am new to TypeScript. I've got a problem with displaying this.state.something inside the render method or assigning it to a variable inside a function.

Have a look at the most important piece of code:

interface State {
    playOrPause?: string;
}

class Player extends React.Component {
    constructor() {
        super();

        this.state = {
            playOrPause: 'Play'
        };
    }

    render() {
        return(
            <div>
                <button
                    ref={playPause => this.playPause = playPause}
                    title={this.state.playOrPause} // in this line I get an error
                    >
                    Play
                </button>
           </div>
        );
    }
}

The errors says: "[ts] Property 'playOrPause' does not exist on type 'ReadOnly<{}>'.

I tried to declare the playOrPause property to be a type of string and it didn't work. What am I missing here to make it work?

You need to declare that your component is using the State interface, it used by Typescript's Generics.

interface IProps {
}

interface IState {
  playOrPause?: string;
}

class Player extends React.Component<IProps, IState> {
  // ------------------------------------------^
  constructor(props: IProps) {
    super(props);

    this.state = {
      playOrPause: 'Play'
    };
  }

  render() {
    return(
      <div>
        <button
          ref={playPause => this.playPause = playPause}
          title={this.state.playOrPause} // in this line I get an error
        >
          Play
        </button>
      </div>
    );
  }
}

In case anyone is wondering how to implement it in functional components with hooks ( not in a class):

const [value, setValue] = useState<number>(0);

useState is a generic function, that means that it can accept a type parameter. This type-parameter will tell TypeScript which types are acceptable for this state.

Using state in react with TypeScript

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