commonStock.js

import React, { useState, useEffect } from 'react';

function useStockHandler(props) {
  const [value, setStockValue] = useState(null);

  function handleStatusChange(latestValue) {
    setStockValue(latestValue);
  }

  useEffect(() => {
    StockAPI.subscribeToStock(props.stock.id, handleStatusChange);

    return () => {
      StockAPI.unsubscribeFromStock(props.stock.id, handleStatusChange);
    };
  });

  return value;
}

NewYorkExchange.js

function NewYorkExchange(props) {
  const stockValue = useStockHandler({id:"NewYork"});

  return (
    <p> The current stock value is {stockValue}</p>
  );
}