As i think i fixed the error being canvas missed <canvas id="pongGame"




And then

Uncaught ReferenceError: onKeyDown is not defined

Javascript functions don't work like that. Eigther store the onKeyXXX-functions in an object (then you can reference them by the objects name) or register them directely in those lines as anonymous functions.

I took a look at your code and after fiddling with it for a while it seems to run fine in chrome. You should store key presses in a buffer though, here's an example:

//a buffer of boolean values, to tell us whether or not the key is down
var keys = [];

//attach your event listeners
document.onkeydown = function(e){
    keys[e.keyCode] = true;
};
document.onkeyup = function(e){
    keys[e.keyCode] = false;
};

//now when you want to check if a key is down just do this
if(keys[40] == true){
    //do something
}