用canvas画三角形的方法

<canvas id="favoriteRectangle" width="30" height="30"></canvas>

<script>

function drawFavoriteRectangle() {
      var ctx = $('#favoriteRectangle')[0].getContext('2d');
      //var ctx = bg.getContext('2d');
      ctx.beginPath();
      ctx.moveTo(0, 20);
      ctx.lineTo(10, 0);
      ctx.lineTo(20, 20);

      ctx.lineWidth = 2;
      ctx.strokeStyle = 'black';
      ctx.stroke();
    }

</script>