accessing object variables in javascript

Question

So, I just started javascript and everything was working fine till i came to objects. This peace of code is supposed to create a bouncing ball in a html canvas with javascript but it doesn't work.

  var canvas = document.getElementById("myCanvas");
  var ctx = canvas.getContext("2d");

//clear

function clear() {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
}

here is my ball object

//ball

var ball = {

x: canvas.width / 2,
getX: function() {
    return this.x;
},
setX: function(a) {
    this.x = a;
},

y: canvas.height / 2,
getY: function() {
    return this.y;
},
setY: function(a) {
    this.y = a;
},

mx: 2,
getMx: function() {
    return this.mx;
},
setMx: function(a) {
    this.my = a;
},
my: 4,

getMy: function() {
    return this.my;
},

setMy: function(a) {
    this.my = a;
},
r: 10,
getR: function() {
    return this.r;
}

};

code to draw my ball

function drawBall()
{
    ctx.beginPath();
    ctx.arc(ball.getX, ball.getY, ball.getR, 0, Math.PI * 2, true);
    ctx.fillStyle = "#83F52C";
    ctx.fill();

}

function circle(x, y, r) {
    ctx.beginPath();
    ctx.arc(x, y, r, 0, Math.PI * 2, true);
    ctx.fillStyle = "#83F52C";
    ctx.fill();
}
//draws ball and updates x,y cords 
function draw() {
    clear();
    drawBall();
    if (ball.getX() + ball.getMx() >= canvas.width || ball.getX()+ ball.getMx() <= 0) {
        ball.setMx(-ball.getMx());
    }
    if (ball.getY() + ball.getMy() >= canvas.height|| ball.getY()+ ball.getMy() <= 0) {
        ball.setMy(-ball.getMy());
    }

    ball.setX(ball.getX() + ball.getMx());
    ball.setY(ball.getY() + ball.getMy());

}

set interval

function init() {
    return setInterval(draw, 10);
}

init();

I take it youre sure you have the canvas there in html?

Alex
– Alex

2012-06-12 22:45:31 +00:00
Commented Jun 12, 2012 at 22:45 — Alex
– Alex, Commented Jun 12, 2012 at 22:45

user1106925 · Accepted Answer · 2012-06-12 22:35:38Z

2

Use this to reference the properties of the object on which the method is invoked.

var ball = {

    x: canvas.width / 2,
    getX: function() {
        return this.x;
    },
    setX: function(a) {
        this.x = a;
    },

    y: canvas.height / 2,
    getY: function() {
        return this.y;
    },
    setY: function(a) {
        this.y = a;
    },

    mx: 2,
    getMx: function() {
        return this.mx;
    },
    my: 4,
    getMy: function() {
        return this.my;
    },
    r: 10,
    getR: function() {
        return this.r;
    }
};

Also, to call methods, you need to use ().

ctx.arc(ball.getX(), ball.getY(), ball.getR(), 0, Math.PI * 2, true);

Otherwise you're passing the method instead of the result of calling the method.

answered Jun 12, 2012 at 22:35

user1106925

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