miterLimit:

your browser does not support the canvas tag

var c = document.getElementById('myCanvas');
var ctx = c.getContext('2d');
ctx.lineWidth = 10;
ctx.lineJoin = 'miter';
ctx.miterLimit = 10;
ctx.moveTo(20, 20);
ctx.lineTo(50, 30);
ctx.lineTo(20, 40);
ctx.stroke();