Canvas' fps unintentionally speeding up?

I am messing around with the canvas element based off of tutorials online and have constructed the following page, here.

Here is my markup:

<!DOCTYPE html>

<html lang="en">
    <head>
        <meta charset="utf-8" />
        <title>Canvas Game</title>

        <link rel="stylesheet" href="style.css">

        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
        <script src="script.js"></script>
    </head>
    <body>
    <header>
        <h1>Cool Canvas Bouncing Effect!</h1>
        <p>Which would you like to see bounce around?</p>
        <input id="beachBallButton" type="button" value="Beach Ball" />
        <input id="avatarButton" type="button" value="Avatar" />
    </header>
    <br />
    <canvas id="myCanvas" width="600" height="400">
        Your browser does not support canvas!
    </canvas>
    </body>
</html>

And here is my JavaScript:

$(document).ready(function() {

const FPS;
var x = 0;
var y = 0;
var xDirection = 1;
var yDirection = 1;
var image = new Image();
image.src = null;
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext('2d');

$('#avatarButton').click(function() {
    x = 0;
    y = 0;
    FPS = 30;
    image.src = 'avatar.png';

    setInterval(draw, 1000 / FPS);

    function draw() {
        ctx.clearRect(0,0,canvas.width,canvas.height);
        ctx.fillRect(0,0,canvas.width,canvas.height);
        ctx.drawImage(image, x, y);

        x += 1 * xDirection;
        y += 1 * yDirection;

        if (x >= 500)
        {
            x = 500;
            xDirection = -1;
        }
        else if (x <= 0)
        {
            x = 0;
            xDirection = 1;
        }

        if (y >= 300)
        {
            y = 300;
            yDirection = -1;
        }
        else if (y <= 0)
        {
            y = 0;
            yDirection = 1;
        }
    }
});

$('#beachBallButton').click(function() {
    x = 0;
    y = 0;
    FPS = 60;
    image.src = 'beachball.png';

    setInterval(draw, 1000 / FPS);

    function draw() {
        ctx.clearRect(0,0,canvas.width,canvas.height);
        ctx.fillRect(0,0,canvas.width,canvas.height);
        ctx.drawImage(image, x, y);

        x += 5 * xDirection;
        y += 5 * yDirection;

        if (x >= 450)
        {
            x = 450;
            xDirection = -1;
        }
        else if (x <= 0)
        {
            x = 0;
            xDirection = 1;
        }

        if (y >= 250)
        {
            y = 250;
            yDirection = -1;
        }
        else if (y <= 0)
        {
            y = 0;
            yDirection = 1;
        }
    }
});

});

Now, say you click on the Avatar button, and then click on the Beach Ball afterwards, the FPS is sped up. However, I reset the FPS variable within the click functions of both functions. I also reset the x and y variables.

Additionally, I could just keep clicking the same button and the speed will increase dramatically as well.

Could someone help explain why this is happening?

Thanks!

Solution

MMM..... const FPS; <--- What is this?

As far as I remember, there is no constants in javascript. Anyway, if it is a constant, why you try to set it's value later several times? I think that this statement fails and the first time you set the FPS, you create a global var, and later this global var is shared by all the functions.

Also, you don't use clearInterval, and you are invoking a new setInterval every time you make a click, so if you click 3 times in "beachBallButton", you will have 3 setIntervals functions running, each of them executing the code. This is likely to cause the "speed up".