Detailed explanation of how to use JS to achieve the effect of PS eraser scratch card in canvas

 <div class="box" id="bb">
        <canvas id="cas" width="1366" height="651"></canvas>
    </div>
 
    <script type="text/javascript" charset="utf-8">
        var canvas = document.getElementById("cas"),
            ctx = canvas.getContext("2d");
        var x1, y1, a = 30,
            timeout, totimes = 100,
            jiange = 30;
        canvas.width = document.getElementById("bb").clientWidth;
        canvas.height = document.getElementById("bb").clientHeight;
        var img = new Image();
        img.src = "sha.jpg";
        img.onload = function() {
            ctx.drawImage(img, 0, 0, canvas.width, canvas.height)
            //ctx.fillRect(0,0,canvas.width,canvas)
            tapClip()
        }
 
        //Achieve the erasing effect by modifying globalCompositeOperation function tapClip() {
            var hastouch = "ontouchstart" in window ? true : false,
                tapstart = hastouch ? "touchstart" : "mousedown",
                tapmove = hastouch ? "touchmove" : "mousemove",
                tapend = hastouch ? "touchend" : "mouseup";
 
            ctx.lineCap = "round";
            ctx.lineJoin = "round";
            ctx.lineWidth = a * 2;
            ctx.globalCompositeOperation = "destination-out";
 
            canvas.addEventListener(tapstart, function(e) {
                clearTimeout(timeout)
                e.preventDefault();
 
                x1 = hastouch ? e.targetTouches[0].pageX : e.clientX - canvas.offsetLeft;
                y1 = hastouch ? e.targetTouches[0].pageY : e.clientY - canvas.offsetTop;
 
                ctx.save();
                ctx.beginPath()
                ctx.arc(x1, y1, 1, 0, 2 * Math.PI);
                ctx.fill();
                ctx.restore();
 
                canvas.addEventListener(tapmove, tapmoveHandler);
                canvas.addEventListener(tapend, function() {
                    canvas.removeEventListener(tapmove, tapmoveHandler);
 
                    timeout = setTimeout(function() {
                        var imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
                        var dd = 0;
                        for (var x = 0; x < imgData.width; x += jiange) {
                            for (var y = 0; y < imgData.height; y += jiange) {
                                var i = (y * imgData.width + x) * 4;
                                if (imgData.data[i + 3] > 0) {
                                    dd++
                                }
                            }
                        }
                        if (dd / (imgData.width * imgData.height / (jiange * jiange)) < 0.4) {
                            canvas.className = "noOp";
                        }
                    }, totimes)
                });
 
           
                canvas.addEventListener(tapmove, tapmoveHandler);
                canvas.addEventListener(tapend, function() {
                    canvas.removeEventListener(tapmove, tapmoveHandler);
 
                    timeout = setTimeout(function() {
                        var imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
                        var dd = 0;
                        for (var x = 0; x < imgData.width; x += jiange) {
                            for (var y = 0; y < imgData.height; y += jiange) {
                                var i = (y * imgData.width + x) * 4;
                                if (imgData.data[i + 3] > 0) {
                                    dd++
                                }
                            }
                        }
 
                    }, totimes)
 
                });
 
            
        }
    </script>
    <script type="text/javascript">
        window.setTimeout('CountDown()', 100);
        // End
    </script>