Question

Game over

The most obvious solution would be to make lives a variable that lives in runGame and is thus visible to the startLevel closure.

Another approach, which fits nicely with the spirit of the rest of the function, would be to add a second parameter to startLevel that gives the number of lives. When the whole state of a system is stored in the arguments to a function, calling that function provides an elegant way to transition to a new state.

In any case, when a level is lost, there should now be two possible state transitions. If that was the last life, we go back to level zero with the starting amount of lives. If not, we repeat the current level with one less life remaining.

Pausing the game

An animation can be interrupted by returning false from the function given to runAnimation . It can be continued by calling runAnimation again.

To communicate that the animation should be interrupted to the function passed to runAnimation so that it can return false , you can use a variable that both the event handler and that function have access to.

When finding a way to unregister the handlers registered by trackKeys , remember that the exact same function value that was passed to addEventListener must be passed to removeEventListener to successfully remove a handler. Thus, the handler function value created in trackKeys must be available to the code that unregisters the handlers.

You can add a property to the object returned by trackKeys , containing either that function value or a method that handles the unregistering directly.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.