lineJoin <br>
<canvas id="test-canvas" width="800" height="600" style="border:1px solid red;"></canvas>

<script>
var canvas = document.getElementById('test-canvas');
var ctx = canvas.getContext('2d');
//style
ctx.lineWidth=10;
ctx.lineJoin="round"; //线条连接处是圆形的
ctx.lineJoin="miter"; //尖角
ctx.lineJoin="bevel"; //斜劈

ctx.beginPath()
ctx.lineTo(50,50)
ctx.lineTo(300,50)
ctx.lineTo(300,300)

ctx.closePath();
ctx.stroke();
</script>