<canvas id="mycanvas" width="500" height="500">您的瀏覽器不支持canvas</canvas>

canvas{ border: 1px solid black;}

var canvas = document.getElementById("mycanvas");        var context = canvas.getContext("2d");        var x1 = 100;        var y1 = 100;        var x2 = 400;        var y2 = 400;        draw();        function draw(){            //畫半透明的線            context.beginPath();            context.moveTo(500,0);            context.lineTo(0,500);            context.strokeStyle = "rgba(0,0,0,0.3)";            context.lineWidth = 10;            context.stroke();            //畫連接線            context.beginPath();            context.moveTo(0,500);            context.lineTo(x1,y1);            context.lineWidth = 2;            context.strokeStyle = "black";            context.stroke();            context.beginPath();            context.moveTo(500,0);            context.lineTo(x2,y2);            context.lineWidth = 2;            context.strokeStyle = "black";            context.stroke();            //畫紅球            context.beginPath();            context.arc(x1,y1,10,0,Math.PI*2);            context.fillStyle = "orange";            context.fill();            //畫藍球            context.beginPath();            context.arc(x2,y2,10,0,Math.PI*2);            context.fillStyle = "blue";            context.fill();            //畫貝塞爾曲線            context.beginPath();            context.moveTo(0,500);            context.bezierCurveTo(x1,y1,x2,y2,500,0);            context.lineWidth = 5;            context.stroke();        }        //拖動小球做動畫        //判斷是否拖動小球        //如果在小球上就做動畫        canvas.onmousedown = function(e){            var ev = e || window.event;            var x = ev.offsetX;            var y = ev.offsetY;            //判斷是否在紅球上            var dis = Math.pow((x-x1),2) + Math.pow((y-y1),2);            if(dis<100){                console.log("鼠標在紅球上");                canvas.onmousemove = function(e){                    var ev = e || window.event;                    var xx = ev.offsetX;                    var yy = ev.offsetY;                    //清除畫布                    context.clearRect(0,0,canvas.width,canvas.height);                    x1 = xx;                    y1 = yy;                    //重繪制                    draw();                }            }            //判斷鼠標是否在藍球上            var dis = Math.pow((x-x2),2) + Math.pow((y-y2),2);            if(dis<100){                canvas.onmousemove = function(e){                    var ev = e || window.event;                    var xx1 = ev.offsetX;                    var yy1 = ev.offsetY;                    //清除畫布                    context.clearRect(0,0,canvas.width,canvas.height);                    x2 = xx1;                    y2 = yy1;                    //重繪制                    draw();                }            }        }        document.onmouseup =function(){            canvas.onmousemove = " ";        }