Hướng dẫn xây dựng đồng hồ bấm giờ bằng JavaScript

<div class="container">
        <h1 class="timer">
            <span class="minutes">00</span> :
            <span class="seconds">00</span> .
            <span class="milliseconds">000</span>
        </h1>
        <ol class="lapContainer"></ol>
        <div class="bottomPart">
            <div class="btns">  
                <button class="reset">Reset</button>
                <button class="toggle">Start</button>
                <button class="lap">Lap</button>
            </div>
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* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Lato', sans-serif;
}

body {
    height: 100vh;
    background: linear-gradient(to right, #d38312, #a83279);
    display: flex;
}

.container {
    margin: auto;
    width: 500px;
    height: 500px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: center;
    position: relative;
}

.timer {
    font-size: 76px;
    font-weight: lighter;
    color: #fff;
    width: 410px;
    margin-left: 20px;
}

.bottomPart {
    width: 100%;
    height: 200px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: center;
}

.copyright {
    align-self: flex-start;
    color: #fff;
    text-decoration: none;
    font-size: 20pt;
    font-weight: 500;
    text-shadow: 2px 2px 2px #0BB1CE;
}

.copyright:hover{
    text-shadow: #474747 3px 5px 2px, 2px 2px 2px rgba(31,206,40,0);
}

.btns {
    width: 300px;
    height: 100px;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.btns button {
    width: 80px;
    height: 80px;
    background-color: transparent;
    color: #fff;
    border: none;
    border-radius: 50%;
    text-transform: uppercase;
    font-weight: bold;
    cursor: pointer;
    outline: none;
    box-shadow: 0px 20px 30px 0px rgba(0, 0, 0, 0.5);
}

.btns .toggle {
    transition: all 0.3s ease;
    background: rgb(105,201,48);
    background: radial-gradient(circle, rgba(105,201,48,1) 0%, rgba(48,182,40,1) 100%);
    z-index: 1;
}

.btns .toggle:active, .btns .reset:active, .btns .lap:active {
    transform: scale(0.9);
}

.btns .toggle.on {
    background-color: #d92027;
}

.btns .reset, .btns .lap {
    transition: all 0.3s ease;
}

.lapContainer {
    width: 200px;
    height: 150px;
    color: #fff;
    padding-left: 30px;
    overflow: auto;
    display: flex;
    flex-direction: column;
    align-items: center;
}

.lapItem {
    width: 120px;
    height: 30px;
    font-size: 18px;
    border-bottom: 1px solid #fff;
    margin-bottom: 5px;
}

::-webkit-scrollbar {
    width: 15px;
    
}

::-webkit-scrollbar-track {
    background: #f1f1f1;
    border-radius: 10px;
}

::-webkit-scrollbar-thumb {
    background: #ff9234;
    border-radius: 10px;
}

::-webkit-scrollbar-thumb:hover {
    background: #ff7600;
}

Và đoạn Script xử lý sự kiện onload bên dưới

//Main
var timer = document.querySelector('.timer');
var toggleBtn = document.querySelector('.toggle');
var resetBtn = document.querySelector('.reset');
var lapBtn = document.querySelector('.lap');
var lapContainer = document.querySelector('.lapContainer')

var watch = new Stopwatch(timer);

function start() {
    toggleBtn.textContent = 'Stop';
    toggleBtn.classList.toggle("on");
    watch.start();
}

function stop() {
    toggleBtn.textContent = 'Start';
    toggleBtn.classList.toggle("on")
    watch.stop();
}

function stopWhenOn() {
    toggleBtn.textContent = 'Start';
    toggleBtn.classList.toggle("on")
    watch.stop();
    watch.reset();
}

toggleBtn.addEventListener('click', function () {
    watch.isOn ? stop() : start();
});

resetBtn.addEventListener('click', function () {
    watch.isOn ? stopWhenOn() : watch.reset();
    // stop();
});

lapBtn.addEventListener('click', function () {
    watch.isOn ? watch.lapOn() : watch.lapOff();
})

//StopWatch
function Stopwatch(elem) {
    var time = 0;
    var offset;
    var interval;

    function lapOn() {
        var lapTime = lapContainer.appendChild(document.createElement("li"))
        lapTime.innerHTML = elem.textContent;
        lapTime.classList = 'lapItem'
    }

    function lapOff() {
        return;
    }

    function update() {
        if (this.isOn) {
            time += delta();
        }
        elem.textContent = timeFormatter(time);
    }

    function delta() {
        var now = Date.now();
        var timePassed = now - offset;

        offset = now;

        return timePassed;
    }

    function timeFormatter(time) {
        time = new Date(time);

        var minutes = time.getMinutes().toString();
        var seconds = time.getSeconds().toString();
        var milliseconds = time.getMilliseconds().toString();

        if (minutes.length < 2) {
            minutes = '0' + minutes;
        }

        if (seconds.length < 2) {
            seconds = '0' + seconds;
        }

        while (milliseconds.length < 3) {
            milliseconds = '0' + milliseconds;
        }

        var result = minutes + ' : ' + seconds + ' . ' + milliseconds;

        return result;
    }

    this.start = function () {
        interval = setInterval(update.bind(this), 1);
        offset = Date.now();
        this.isOn = true;
    };

    this.stop = function () {
        clearInterval(interval);
        interval = null;
        this.isOn = false;
    };

    this.reset = function () {
        time = 0;
        lapContainer.innerHTML = '';
        interval = null;
        this.isOn = false;
        update();
    };

    this.lapOn = function () {
        lapOn();
    }

    this.lapOff = function () {
        lapOff();
    }

    this.isOn = false;
}