clearRect not working

I'm doing a Pong game in javascript in order to learn making games, and I want to make it object oriented.

I can't get clearRect to work. All it does is draw a line that grows longer. Here is the relevant code:

function Ball(){
    this.radius = 5;
    this.Y = 20;
    this.X = 25;
    this.draw = function() {
        ctx.arc(this.X, this.Y, this.radius, 0, Math.PI*2, true);
        ctx.fillStyle = '#00ff00';
        ctx.fill();
    };
}

var ball = new Ball();

function draw(){
    player.draw();
    ball.draw();
}

function update(){
    ctx.clearRect(0, 0, 800, 400);
    draw();
    ball.X++;
}

I've tried to put the ctx.clearRect part in the draw() and ball.draw() functions and it doesn't work. I also tried fillRect with white but it gives the same results. How can I fix this?