New rule proposal

[hornta]

This rule would report on unreachable code paths in React components.

Example: this.state.bar will never be truthy because there is no setState({ bar: ... }); and will make the code inside the condition unreachable.

Example of incorrect code

class Foo extends React.Component {
  constructor() {
    this.state = {
      bar: false
    };
  }

  render() {
    if(this.state.bar) {
      return 'bar'; // this is unreachable
    }
    return 'foo';
  }
}

function Foo() {
  const [bar, setBar] = useState(false);

  if(bar) {
    return 'bar'; // this is unreachable
  }

  return 'foo';
}

function Foo() {
  const [bar, setBar] = useState(false);

  setBar(null); // setting to null still makes the code unreachable

  if(bar) {
    return 'bar'; // this is unreachable
  }

  return 'foo';
}

Example of correct code

class Foo extends React.Component {
  constructor() {
    this.state = {
      bar: false
    };
  }

  componentDidMount() {
    this.setState({ bar: true }); // setting bar to a known non-falsy value
  }

  render() {
    if(this.state.bar) {
      return 'bar'; 
    }
    return 'foo';
  }
}

function Foo() {
  const [bar, setBar] = useState(false);

  setBar(returnWhat()); // because returnWhat can return a truthy value

  if(bar) {
    return 'bar'; // this may be reachable, we can't know
  }

  return 'foo';
}

[ljharb]

That sounds very difficult to code reliably; since this.setState can be passed any object literal, which wouldn't be statically analyzeable - iow, any setState call that didn't have an object literal in it, or that had spread syntax in it, would make the rule useless for the file.