var canvas = document.getElementById('canvas');
if (canvas.getContext) {
  var ctx = canvas.getContext('2d');

  // 二次贝塞尔曲线
  ctx.beginPath();
  ctx.moveTo(75,25);
  ctx.quadraticCurveTo(25,25,25,62.5);
  ctx.quadraticCurveTo(25,100,50,100);
  ctx.quadraticCurveTo(50,120,30,125);
  ctx.quadraticCurveTo(60,120,65,100);
  ctx.quadraticCurveTo(125,100,125,62.5);
  ctx.quadraticCurveTo(125,25,75,25);
  ctx.stroke();
}