Detect if poly is convex or concave

isConvex

/**
 * Tests if a Polygon shape is convex.
 * Pass an Array of vertices(or points) that have x/y properties
 *
 * @return {Boolean} true is a convex shape
 */
Polygon.prototype.isConvex = function()
{
	if (this.vertices.length < 4)
		return true;

	var sign = false;
	var n = this.vertices.length;
	for(var i= 0; i < n; i++)
	{
		var dx1 = this.vertices[(i + 2) % n].x - this.vertices[(i + 1) % n].x;
		var dy1 = this.vertices[(i + 2) % n].y - this.vertices[(i + 1) % n].y;
		var dx2 = this.vertices[i].x - this.vertices[(i + 1) % n].x;
		var dy2 = this.vertices[i].y - this.vertices[(i + 1) % n].y;
		var zcrossproduct = dx1 * dy2 - dy1 * dx2;
		if(i == 0)
		{
			sign = zcrossproduct > 0;
		} else
		{
			if (sign != (zcrossproduct > 0))
				return false;
		}
	}
	return true;
}