js继承实例

<html>
<head>
<script>
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));
}

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

function Triangle(a,b,c){
    this.points = [a,b,c];
    this.getArea = function(){
        var p = this.getPerimeter();
        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 + side_a,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 myclick(){
    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();
    console.log(t.getPerimeter());
    console.log(t.getArea());
    
    var r = new Rectangle(new Point(200,200),50,100);
    r.draw();
    console.log(r.getArea());
    console.log(r.getPerimeter());
    
    var s = new Square(new Point(130,130),50);
    s.draw();
    console.log(s.getArea());
    console.log(s.getPerimeter());
    
    new Square(p1,200).draw();
}
</script>
</head>
<body>
<canvas height="600" width="800" id="canvas" onclick="myclick()"/>
</body>
</html>