script.js

let [milliseconds, seconds, minutes, hours] = [0, 0, 0, 0];
let timerRef = document.querySelector('.timerDisplay');
let int = null;

document.getElementById('startTimer').addEventListener('click', () => {
    if (int !== null) {
        clearInterval(int); 
    }
    int = setInterval(displayTimer, 10); 
});

document.getElementById('pauseTimer').addEventListener('click', () => {
    clearInterval(int); 
});

document.getElementById('resetTimer').addEventListener('click', () => {
    clearInterval(int);
    [milliseconds, seconds, minutes, hours] = [0, 0, 0, 0]; 
    timerRef.innerHTML = '00 : 00 : 00 : 000'; 
});

function displayTimer() {
    milliseconds += 10; 
    if (milliseconds === 1000) {
        milliseconds = 0;
        seconds++;
        if (seconds === 60) {
            seconds = 0;
            minutes++; 
            if (minutes === 60) {
                minutes = 0;
                hours++;
            }
        }
    }

    let h = hours < 10 ? "0" + hours : hours;
    let m = minutes < 10 ? "0" + minutes : minutes;
    let s = seconds < 10 ? "0" + seconds : seconds;
    let ms = milliseconds < 10 ? "00" + milliseconds : milliseconds < 100 ? "0" + milliseconds : milliseconds;

    timerRef.innerHTML = `${h} : ${m} : ${s} : ${ms}`;
}