Simple ES6 Singleton. This is a simple implementation it doesn't shield you from all the things but good enough in majority of the cases. Simple to reason about as well.

app.js

// In your main file
import ModalStore from './store';

console.log(`Test 1: ${ModalStore.getState().date}`); // Test 1: Fri Nov 02 2018 10:12:02 GMT+0100

setTimeout(() => {
	console.log(`Test 2: ${ModalStore.getState().date}`); // Test 2: Fri Nov 02 2018 10:12:02 GMT+0100
}, 500);

// As you can see even though date is a mutable field we have the same value for the instance

setTimeout(() => {
	ModalStore.setState({date: '2018'});
	console.log(`Test 3: ${ModalStore.getState().date}`); // Test 3: 2018
}, 1000);

Modal.js

const modalHOC = store => {
  return class Modal {
	  constructor(config = {}) {
    }
    myPublicMethod() {
      if(store.getState().isOpen) {
        // Do your magic
      }
    }
  }
}
  
  export default modalHOC;

store.js

class Store {
	constructor() {
		this.state = {
			// Just for testing. You proably want that this to be empty object or having some defaults
			date: new Date()
		};
	}

	getState() {
		return this.state;
	}

	setState(newState) {
		this.state = {
			...this.state,
			...newState
		};
	}
}
const singleton = new Store();

export default singleton;