import { useState, useEffect, useRef } from "react";

export default function App() {
  const [day, setDay] = useState("Monday");
  const prevDay = usePrevious(day);

  const getNextDay = () => {
    if (day === "Monday") {
      setDay("Tuesday")
    } else if (day === "Tuesday") {
      setDay("Wednesday")
    } else if (day === "Wednesday") {
      setDay("Thursday")
    } else if (day === "Thursday") {
      setDay("Friday")
    } else if (day === "Friday") {
      setDay("Monday")
    }
  }

  return (
    <div style={{padding: "0 20px"}}>
      <h1>
        Today is: {day}<br />
        {
          prevDay && (
            <span>Previous work day was: {prevDay}</span>
          )
        }
      </h1>
      <button onClick={getNextDay}>
        Get next day
      </button>
    </div>
  );
}

function usePrevious(val) {
}