I was trying to see how many times the code re-render and hence used a reference Hook to check it. Though the reference variable was updating when I was using shorthand operator it was not working using assignment operator and is printing NaN.

import { useState, useRef } from 'react';

export default function Stopwatch() {
  const [startTime, setStartTime] = useState(null);
  const [now, setNow] = useState(null);
  const ref = useRef(0);

    //ref.current=ref.current+1;
      ref.current+=1;
  function handleStart() {
    // Start counting.
    setStartTime(Date.now());
    setNow(Date.now());

    setInterval(() => {
      // Update the current time every 10ms.
      setNow(Date.now());
    }, 10);
  }

  let secondsPassed = 0;
  if (startTime != null && now != null) {
    secondsPassed = (now - startTime) / 1000;
  }

  return (
    <>
      <h1>Time passed: {secondsPassed.toFixed(3)}</h1>
      <button onClick={handleStart}>
        Start
      </button>
        <p>{ref.current}</p>
    </>
  );
}

Anything I am missing while assigning this?