script.js

let[milliseconds, seconds, minutes, hours] = [0, 0, 0, 0];
let timeRef = document.querySelector(".timer-display");
let int = null;

document.getElementById("start-timer").addEventListener("click",
    ()=>{
        if(int !==null){
            clearInterval(int);
        }
        int= setInterval(displayTimer, 10);
    });

document.getElementById("pause-timer").addEventListener("click",
        ()=>{
            clearInterval(int);
        });

document.getElementById("reset-timer").addEventListener("click",
            ()=>{
                clearInterval(int);
                [milliseconds, seconds, minutes, hours] = [0, 0, 0, 0];
                timeRef.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;

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