and my script is :

const starting = 3;
let time = starting;

countdownEl = document.getElementById('countdown');

setInterval(updateCountdown,1000);

function updateCountdown(){
    let seconds  = time;
    countdownEl.innerHTML = `${seconds}`;
    if (time > 0) {
        time--;
    }
    
    if (time == 0) {
        window.location="{{ url_for('disinfect') }}";
    }
}