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.

In my case ( working with TypeScript, and the state value was actually a boolean ) I've had the same problem, I've fixed it by passing the state value I wanted to mark as output to String():

import React, { Component } from 'react';

interface ITestProps {
  name: string;
}

interface ITestState {
  toggle: boolean;
}

class Test extends Component<ITestProps, ITestState> {
  constructor(props: ITestProps) {
    super(props);

    this.state = {
      toggle: false,
    };

    this.onClick = this.onClick.bind(this);
  }

  onClick() {
    this.setState((previousState, props) => ({
      toggle: !previousState.toggle,
    }));
  }

  render() {
    return (
      <div>
        Hello, {this.props.name}!
        <br />
        Toggle state is: {String(this.state.toggle)}
      </div>
    )
  }
}

Just declare interface or type with property, types, and annotate it to state. the ? mean optional:

interface ITestProps {}

interface ITestState {
  playOrPause?: string;
}

class Player extends React.Component<ITestProps, ITestState> {

  state = {
     playOrPause: 'Play'
  };
  

  render() {
    return // your code here
}

You can add more value as per your need to interface above if then you need the same state to pass it to child component you just need to create a file with .d.ts and you should be good to go!