Question

A vector type

Your solution can follow the pattern of the Rabbit constructor from this chapter quite closely.

Adding a getter property to the constructor can be done with the Object.defineProperty function. To compute the distance from (0, 0) to (x, y), you can use the Pythagorean theorem, which says that the square of the distance we are looking for is equal to the square of the x-coordinate plus the square of the y-coordinate. Thus, √(x² + y²) is the number you want, and Math.sqrt is the way you compute a square root in JavaScript.

Another cell

You’ll have to store all three constructor arguments in the instance object. The minWidth and minHeight methods should call through to the corresponding methods in the inner cell but ensure that no number less than the given size is returned (possibly using Math.max ).

Don’t forget to add a draw method that simply forwards the call to the inner cell.

Sequence interface

One way to solve this is to give the sequence objects state, meaning their properties are changed in the process of using them. You could store a counter that indicates how far the sequence object has advanced.

Your interface will need to expose at least a way to get the next element and to find out whether the iteration has reached the end of the sequence yet. It is tempting to roll these into one method, next , which returns null or undefined when the sequence is at its end. But now you have a problem when a sequence actually contains null . So a separate method (or getter property) to find out whether the end has been reached is probably preferable.

Another solution is to avoid changing state in the object. You can expose a method for getting the current element (without advancing any counter) and another for getting a new sequence that represents the remaining elements after the current one (or a special value if the end of the sequence is reached). This is quite elegant—a sequence value will “stay itself” even after it is used and can thus be shared with other code without worrying about what might happen to it. It is, unfortunately, also somewhat inefficient in a language like JavaScript because it involves creating a lot of objects during iteration.

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.