code: function shapeArea(vertexes) {
  	var area = 0,
  		i,
  		j;
  	for (i = 0; i < vertexes.length; i++) {
  		j = (i + 1) % vertexes.length;
  		area += vertexes[i].x * vertexes[j].y - vertexes[j].x * vertexes[i].y;
  	}
  	return area / 2;
  }
docs: Shoelace algorithm -- http://en.wikipedia.org/wiki/Shoelace_formula