Tried to commit score but a problem occurred and few blank rows left - so they indicate some local change in the file and the code I tried was:
в helpers:
Screen.prototype.drawScore = function(text){
    this.ctx.font="20px Verdana";
    this.ctx.fillStyle = '#f00';
    this.ctx.fillText(text,this.width-50,this.height-50);
};
в game:
player = {
            sprite: taSprite,
            score:0,
            x: (screen.width - taSprite.w) / 2,
            y: screen.height - (30 + taSprite.h)
        };
: -------------------
 aliens.splice(j, 1);
                    j--;
                    len2--;
                    bullets.splice(i, 1);
                    i--;
                    len--;
                    player.score+=10;
 -----------------------------------
 //draw score
        screen.ctx.save();
        screen.drawScore(player.score);
        screen.ctx.restore();