There is no silver bullet to avoid a cheater in general. This is particularly true with games written in JavaScript, since their source code is so easy to access. Of course, you can obfuscate your code, but that will only slow down someone really motivated to figure out your code. There are, however, a few other techniques that you can use to make it more difficult or less efficient to cheat in your game.
The safest way to prevent cheating is to move things on the server side. If you remember, that's exactly what we did with the fight mechanism in our MMORPG in Chapter 7, Making a Multiplayer Game. To apply the same paradigm to a platformer would effectively mean transmitting every keystroke to the server and letting the server decide the resulting position for the player.
In most cases, this is not a realistic solution. But you can still use the server-side logic to validate the score submitted by the player. You can have a series of invisible...