SOURCE

let countDown = localStorage.getItem("countDown");
let countDownDom = null;

const handleCountDown = () => {
    if (!countDown) localStorage.setItem("countDown", 5);
    if (countDown <= 0) {
        countDown = 0;
        localStorage.setItem("countDown", 5);
    }
    countDown = localStorage.getItem("countDown");
    //   countDownDom = document.querySelector(".countdown");
    countDownDom.innerHTML = countDown;
    let timer = setInterval(function () {
        countDown--;
        countDownDom.innerHTML = countDown;
        if (countDown < 0) {
            clearInterval(timer);
            countDownDom.innerHTML = '';
            localStorage.removeItem("countDown");
        }
    }, 1000);
};

window.onload = function () {
    countDownDom = document.querySelector(".countdown");
    if (countDown > 0) {
        countDownDom.innerHTML = countDown;
        handleCountDown();
    }
    //   countDownDom.innerHTML = 60;
    //   handleCountDown();
};

window.onbeforeunload = function () {
    localStorage.setItem("countDown", countDown);
};

function handleStart() {
    handleCountDown();
}

<div>
	<span class="countdown"></span>
    <button onclick="handleStart()">countDown</button>
</div>