index.html

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #canvas {
            display: block;
            width: 700px;
            height: 400px;
            /*border: 5px solid #afafaf;*/
            box-shadow: 10px 10px 10px #afafaf;
        }
        #clearBtn {
            background: red;
            padding: 20px 30px;
            display: block;
        }
        section {
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            align-content: center;
        }
    </style>
</head>
<body>
<section>
    <canvas id="canvas" width="700" height="400"></canvas>
    <button id="clearBtn">Clear</button>
</section>

<script>
    // Get the canvas element and context
    const canvas = document.getElementById('canvas');
    const ctx = canvas.getContext('2d');

    // Set the stroke style
    ctx.strokeStyle = 'black';
    ctx.lineWidth = 2;
    ctx.lineCap = 'round';

    // Initialize variables
    let drawing = false;
    let lastX = 0;
    let lastY = 0;

    // Event listeners
    canvas.addEventListener('mousedown', startDraw);
    canvas.addEventListener('mousemove', draw);
    canvas.addEventListener('mouseup', endDraw);
    canvas.addEventListener('mouseout', endDraw);
    document.getElementById('clearBtn').addEventListener('click', clearCanvas);

    // Functions
    function startDraw(e) {
        drawing = true;
        lastX = e.offsetX;
        lastY = e.offsetY;
    }

    function draw(e) {
        if (!drawing) return;
        ctx.beginPath();
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(e.offsetX, e.offsetY);
        ctx.stroke();
        lastX = e.offsetX;
        lastY = e.offsetY;
    }

    function endDraw() {
        drawing = false;
    }

    function clearCanvas() {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
    }
</script>
</body>
</html>