<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 = " ";
        }