Player movement in vertical shooter.

gistfile1.js

    var player = jam.Sprite(400, 500);
    player.setImage("ship.png");
    player.update = jam.extend(player.update, function(elapsed){
        player.velocity.x = 0;
        player.velocity.y = 0;
        if( jam.Input.keyDown("UP") ){
            player.velocity.y = -250;
        }
        if( jam.Input.keyDown("DOWN") ){
            player.velocity.y = 250;
        }
        if( jam.Input.keyDown("LEFT") ){
            player.velocity.x = -250;
        }
        if( jam.Input.keyDown("RIGHT") ){
            player.velocity.x = 250;
        }
    });
    game.add(player);