//My approach

document.getElementById("countDownButton").onclick = function() {
    let i = Number.parseInt(document.getElementById("counter").innerHTML);
    i--;
    document.getElementById("counter").innerHTML = i;
}

document.getElementById("resetButton").onclick = function() {
    document.getElementById("counter").innerHTML = 0;
}

document.getElementById("countUpButton").onclick = function() {
    let i = Number.parseInt(document.getElementById("counter").innerHTML);
    i++;
    document.getElementById("counter").innerHTML = i;
}