Why aren't the movements working?
I need help, I thought I was doing everything right, but the movements of the ball aren't working.
There is a lot of reasons your movement isn't working, change
if(event.key == "ArrowLeft"), change
var interval into
var interval = null;, change
if(event.key == "ArrowRight"). this should work :)
(==) performs type coercion, whereas (===) doesn't.
Usually type coercion for equality is unexpected or otherwise unwanted.