window.onload = function () {
            //得到2D渲染上下文
            var context = document.getElementById("myCanvas").getContext("2d");

            var x = 50, y = 50, //原点坐标
                radius = 50, //半径
                startAngle = 0, //起始弧度
                endAngle = Math.PI * 2, 终止弧度
            anticlockwise = false; //是否按逆时针
            //0度为沿x水平方向

            //画一个圆
            context.beginPath();
            context.arc(x, y, radius, startAngle, endAngle, anticlockwise);
            context.fillStyle = "red";
            context.fill();

            //画1/4个圆
            context.beginPath();
            x = 150, y = 150, startAngle = 0, endAngle = Math.PI / 2;
            context.arc(x, y, radius, startAngle, endAngle, anticlockwise);
            context.lineTo(x, y);//需要连接原点形成半圆封闭区
            context.closePath(); //闭合路径,这里填充会自动闭合,也可以不写
            context.fillStyle = "blue";
            context.fill();

            //弧形填充
            context.beginPath();
            x = 160, y = 50;
            context.arc(x, y, radius, startAngle, endAngle, true);
            context.fillStyle = "green";
            context.fill();