context.quadraticCurveTo(controlX, controlY, endingX, endingY)
绘制从当前笔位置开始到给定结束坐标的二次曲线。另一个给定的控制坐标确定曲线的形状（弯曲度）。
<!doctype html>
<html>
<head>
<style>
body{ background-color:white; }
#canvas{border:1px solid red; }
</style>
<script>
window.onload=(function(){
// 获取对canvas元素及其上下文的引用
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// 论点
var startX=25;
var startY=70;
var controlX=75;
var controlY=25;
var endX=125;
var endY=70;
// A quadratic curve drawn using "moveTo" and "quadraticCurveTo" commands
ctx.beginPath();
ctx.moveTo(startX,startY);
ctx.quadraticCurveTo(controlX,controlY,endX,endY);
ctx.stroke();
}); // 结束window.onload
</script>
</head>
<body>
<canvas id="canvas" width=200 height=150></canvas>
</body>
</html>