в помеченной строке выдается ошибка p1 undefined Почему?

Код:

<html>
 <head>
  </head>
 <body>
  <canvas height="600" width="800" id="canvas" />

 <script type="text/javascript">
 function Point(x,y)
 {
	 this.x = x;
	 this.y = y;
 }

 function Line(p1,p2)
 {
	 this.p1 = p1;
	 this.p2 = p2;
	 this.length = Math.sqrt(Math.pow(p1.x - p2.x, 2) + Math.pow(p1.y - p2.y, 2));  /////////error here********************
 }


 function Shape()
 {
	 this.points = [];   /////////////
	 this.lines = [];    //////////////
	 this.init();
 }


 Shape.prototype = {
	 constructor:Shape,
	 init: function()
	 {
		 if(typeof this.context === 'undefined')
		 {
			 var canvas = document.getElementById('canvas');
			 Shape.prototype.context = canvas.getContext('2d');
		 }
	 },
	draw: function()
	 {
		var ctx = this.context;
		ctx.strokeStyle = this.getColor();
		ctx.beginPath();
		ctx.moveTo(this.points[0].x, this.points[0].y);
		for(var i=1; i<this.points.length; i++)
			ctx.lineTo(this.points[i].x, this.points[i].y);
		
		ctx.closePath();
		ctx.stroke();
	 },
	getColor: function()
	 {
		var rgb = [];
		for(var i=0; i<3; i++)
			rgb[i] = Math.round(255*Math.random())
		return 'rgb('+ rgb.join(',') + ')';
	 },
	getLines: function()					///////////////
	 {
		 if(this.lines.length > 0)
			 return this.lines;
		 var lines = [];
		 for(var i=0; this.points.length; i++)
			 lines[i] = new Line(this.points[i], (this.points[i+1])?this.points[i+1]:this.points[0]);
		 this.lines = lines;
		 this.lines;
		 return lines;
	 },
	getArea: function(){},
	getPerimeter: function()				////////////////
	 {
		 var lines = this.getLines();
		 var perim = 0;
		 for(var i=0; i<=lines.length; i++)
			 perim +=lines[i].length;
	 }
 }

 function Triangle(a,b,c)
 {
	 this.points = [a,b,c];
	 this.getArea = function()
	 {
		 var p = this.getPerimeter();
		 var s = p/2;
		 return Math.sqrt(s*(s-this.lines[0].length)*(s-this.lines[1].length)*(s-this.lines[2].length));
	 }
 }

 function Rectangle(p, side_a, side_b)
 {
	 this.points = [p, new Point(p.x + p.y), new Point(p.x + side_a, p.y + side_b), new Point(p.x, p.y + side_b)];
	 this.getArea = function()
	 {
		return side_a * side_b;
	 }
 }


 function Square(p, side)
 {
	 Rectangle.call(this, p, side, side);
 }

(function(){
	var s =new Shape();
	Triangle.prototype = s;
	Rectangle.prototype = s;
	Square.prototype = s;
})()

var p1 = new Point(100,100);
var p2 = new Point(300,100);
var p3 = new Point(200,0);

var t = new Triangle(p1, p2, p3);
t.draw();
t.getPerimeter(); ///////////////
//t.getArea();

 
 </script>

 </body>
</html>