Cool! I love the concept and the simple graphics!
1. I would suggest having a "new game" button though, so I don't have to refresh my tab all of the time.
2. Also, change the content of your title tag on line 7, so that it doesn't just say "repl.it."
3. In addition to that, if possible, make it less impossible. This game is really hard.
4. Thirdly, if you can, add health bars.
5. Lastly, make it so that I can get a high score.

Here's a quick way to get a high score:

if (window.localStorage.get("visited")) {
    //The player has been here before
} else {
    //The player has not been here before
    window.localStorage.setItem("visited", "true");
    window.localStorage.setItem("highscore", "0");
}

function checkScore() {
    if (score>JSON.parse(window.localStorage.get("highscore"))) {
        window.localStorage.setItem("highscore", JSON.stringify(score));
    }
}

setInterval(checkScore, 30);