Something like the following

• define the state in the constructor and initialize it with the value you use in the useState
• create a setIsAuth method that sets the state to the new value passed

class YourComponent extends React.Component{
 
  constructor(props){
    super(props);
    this.state = {
      isAuth: localStorage.getItem("isAuth")
    }
  }

  setIsAuth = (newValue) => {
    this.setState({
      isAuth: newValue
    });
  }

  render() {
    ...
  }
}