In the update calls, we have been incrementing the position of the ship sprite by 2. In real life, however, you may want to store that value as a global constant or a centralized setting. In this case, changing the overall speed means that we need to update a single constant or variable, instead of refactoring the whole game.

We have already moved the screen size into constants; let's do the same with the speed:

1. Introduce a new constant called shipSpeed and set its value to 2:

      const screenWidth = 800;
      const screenHeight = 600;
      const shipSpeed = 2;

2. Now, update all of the existing code and use the shipSpeed constant in all of the places you need to increment or decrement the position of the ship, as shown in the following code:

      function update() {
        // RIGHT button
        if (cursors.right.isDown) {
          ship.x += shipSpeed;
          if (ship.x >= screenWidth) {
            ship.x = 0;
          }
          ship.flipX = false;
        }
        // LEFT button
        else if (cursors.left.isDown) {
          ship.x -= shipSpeed;
          if (ship.x <= 0) {
            ship.x = screenWidth;
          }
          ship.flipX = true;
        }
        // UP button
        else if (cursors.up.isDown) {
          ship.y -= shipSpeed;
          if (ship.y <= 0) {
            ship.y = screenHeight;
          }
        }
        // DOWN button
        else if (cursors.down.isDown) {
          ship.y += shipSpeed;
          if (ship.y >= screenHeight) {
            ship.y = 0;
          }
        }
      }

As you can see, you now have a single place that holds the speed of the ship. Any changes that are made to the shipSpeed constant will be reflected in all of the code blocks since we're not refactoring the code in multiple places. 

3. Now, change the speed to 4 and see what happens:

      const shipSpeed = 4;

Notice how the ship is now two times faster than before when it moves in all four directions. You can experiment more and either increase or decrease the speed of the ship until you find a value that provides the best and smooth movement behavior.