<canvas id="canvas" width="600" height="400" style="border: 1px solid black;"> </canvas>    <script type="text/javascript">        let canvas = document.getElementById('canvas'),            ctx = canvas.getContext('2d'),            canvasLeft = canvas.getBoundingClientRect().left, //getBoundingClientRect()獲取元素位置            canvasTop = canvas.getBoundingClientRect().top;        let imageData; //記錄圖像數據        let start = new Map([['x',null],['y',null]]);        let drag = false;//記錄是否處于拖動狀態        canvas.onmousedown = function (e) {            let pos = positionInCanvas(e, canvasLeft, canvasTop);            start.set('x', pos.x);            start.set('y', pos.y);            drag = true;            //記錄imageData            imageData = ctx.getImageData(0,0,canvas.width,canvas.height);        }        canvas.onmousemove = function (e) {            if(drag === true){               let pos = positionInCanvas(e, canvasLeft, canvasTop);               //相當于把掃描出來的線都擦掉，重新畫               ctx.putImageData(imageData, 0, 0);               ctx.beginPath();               ctx.moveTo(start.get('x'), start.get('y'));               ctx.lineTo(pos.x, pos.y);               ctx.stroke();            }        }        canvas.onmouseup = function  (e) {            drag = false;        }        function positionInCanvas (e, canvasLeft, canvasTop) {//獲取canvas中鼠標點擊位置         return {                  x:e.clientX - canvasLeft,                  y:e.clientY - canvasTop          }               }        </script>