未定義のJavaScriptとOOPのプロパティを読み取ることができません

「gameLoop」というプロトタイプメソッドを持つ「Game」というオブジェクト（関数）があります。このループを一定の間隔で呼び出す必要があるので、これを実行しようとします。

setInterval(game.gameLoop,setIntervalAmount);

しかし、受け取る "TypeError: Cannot read property 'clearRect' of undefined(…)"

プロトタイプメソッドは次のとおりです。

Game.prototype.gameLoop = function()
{
    this.context.clearRect(0,0,this.canvas.width, this.canvas.height);
    this.context.save();


    this.context.translate(this.canvas.width/2, this.canvas.height/2);
    this.context.scale(this.camera.scale,this.camera.scale);
    this.context.rotate(this.camera.rotate);
    this.context.translate(this.camera.x,this.camera.y);


    for(var i=0;i<this.objects.length;i++)
    {
        this.objects[i].updateSprite();
        this.objects[i].drawSprite(this.context);         
    }
    this.context.restore();
}

私はまだJavascriptのオブジェクト指向プログラミングを理解するのに苦労しています。関数が単なる通常の関数であり、ゲームオブジェクトを渡した作業バージョンがありました。何か案は？

ちなみに、ここに役立つかもしれないいくつかの追加のコードがあります。

function Sprite(imgg,w,h)
{
    this.img = imgg;
    this.x = 350;//Math.random()*700;
    this.y = 350;//Math.random()*700;
    this.vx = 0;//Math.random()*8-4;
    this.vy = 0;//Math.random()*8-4;
    this.width = w;
    this.height = h;
    this.rotatespeed = 0.01;
    this.rotate = 40;

}
Sprite.prototype.drawSprite = function(ctx)
{

    ctx.save();


    ctx.translate(this.x,this.y);
    ctx.rotate(this.rotate);


    ctx.drawImage(this.img,0,0,this.img.width,this.img.height,-this.width/2,-this.height/2,this.width,this.height);


    ctx.restore();
}
Sprite.prototype.updateSprite = function()
{
    this.x += this.vx;
    this.y += this.vy;

    this.rotate += this.rotatespeed;


    if(this.x > 700)
        this.vx = -this.vx;
    if(this.x < 0)
        this.vx = -this.vy;

    if(this.y > 700)
        this.vy = -this.vy;
    if(this.y < 0)
        this.vy = -this.vy;
}
Sprite.prototype.mouseEventListener = function(evt, type)
{
    console.log("Hello");
}


//------------------------------------------
//GLOBAL VARIALBES

var setIntervalAmount = 30;
var scrollAmount = 0.5;
var game;
function Game()
{
    this.camera = new Object();
    this.camera.x = -350;
    this.camera.y = -350;
    this.camera.scale = 1;

    this.camera.rotate = 0;

    this.canvas = document.createElement("canvas");
    document.body.appendChild(this.canvas);
    this.canvas.id="mycanvas";
    this.canvas.width = 700;
    this.canvas.height = 700;
    this.context = this.canvas.getContext("2d");    

    var ctx = this.context;

    ctx.canvas.addEventListener('mousemove', function(event){
        var mouseX = event.clientX - ctx.canvas.offsetLeft;
        var mouseY = event.clientY - ctx.canvas.offsetTop;

        var canvasX = mouseX * ctx.canvas.width / ctx.canvas.clientWidth;
        var canvasY = mouseY * ctx.canvas.height / ctx.canvas.clientHeight;


        //console.log(canvasX+" | "+canvasY);

    });

    this.objects = new Array();  


}
Game.prototype.handleMouse = function(evt,type)
{
    for(var i=0;i<this.objects.length;i++)
    {
        this.objects[i].mouseEventListener(evt,type);
    }
};

Game.prototype.gameLoop = function()
{
    this.context.clearRect(0,0,this.canvas.width, this.canvas.height);
    this.context.save();


    this.context.translate(this.canvas.width/2, this.canvas.height/2);
    this.context.scale(this.camera.scale,this.camera.scale);
    this.context.rotate(this.camera.rotate);
    this.context.translate(this.camera.x,this.camera.y);


    for(var i=0;i<this.objects.length;i++)
    {
        this.objects[i].updateSprite();
        this.objects[i].drawSprite(this.context);         
    }
    this.context.restore();
}


/*Game.prototype.drawGame = function()
{
    var gameLoop = setInterval(function(){

        this.context.clearRect(0,0,this.canvas.width, this.canvas.height);
        this.context.save();


        this.context.translate(this.canvas.width/2, this.canvas.height/2);
        this.context.scale(this.camera.scale,this.camera.scale);
        this.context.rotate(this.camera.rotate);
        this.context.translate(this.camera.x,this.camera.y);


        for(var i=0;i<this.objects.length;i++)
        {
            this.objects[i].updateSprite();
            this.objects[i].drawSprite(this.context);         
        }
        this.context.restore();

    },setIntervalAmount); 

}*/



function mouseWheelListener() 
{
    var evt = window.event;
    console.log(evt.wheelDelta);
    if(evt.wheelDelta < 0)
        game.camera.scale /= (1+scrollAmount);
    else 
        game.camera.scale *= (1+scrollAmount);

}
function mouseDownListener()
{
    var evt = window.event;
    var type = "down"
    game.handleMouse(evt,type);
}
function mouseUpListener()
{
    var evt = window.event;
    var type = "up"
    game.handleMouse(evt,type);
}
function mouseMoveListener()
{
    var evt = window.event;
    var type = "move"
    game.handleMouse(evt,type);
}

//------------------

window.addEventListener('load',function(event){startgame();});

var dog = new Image();
dog.src = "grid.gif";

function startgame()
{
    game = new Game();

    for(var i=0;i<1;i++)
        game.objects.push(new Sprite(dog,250,250));


    setInterval(game.gameLoop,setIntervalAmount);

    document.getElementById("mycanvas").addEventListener("wheel", mouseWheelListener);
    document.getElementById("mycanvas").addEventListener("mousedown", mouseDownListener);
    document.getElementById("mycanvas").addEventListener("mouseup", mouseUpListener);
    document.getElementById("mycanvas").addEventListener("mousemove", mouseMoveListener);
}