Question

Mother-child age difference

Because not all elements in the ancestry array produce useful data (we can’t compute the age difference unless we know the birth date of the mother), we will have to apply filter in some manner before calling average . You could do it as a first pass, by defining a hasKnownMother function and filtering on that first. Alternatively, you could start by calling map and in your mapping function return either the age difference or null if no mother is known. Then, you can call filter to remove the null elements before passing the array to average .

Historical life expectancy

The essence of this example lies in grouping the elements of a collection by some aspect of theirs—splitting the array of ancestors into smaller arrays with the ancestors for each century.

During the grouping process, keep an object that associates century names (numbers) with arrays of either person objects or ages. Since we do not know in advance what categories we will find, we’ll have to create them on the fly. For each person, after computing their century, we test whether that century was already known. If not, add an array for it. Then add the person (or age) to the array for the proper century.

Finally, a for / in loop can be used to print the average ages for the individual centuries.

Every and then some

The functions can follow a similar pattern to the definition of forEach at the start of the chapter, except that they must return immediately (with the right value) when the predicate function returns false—or true. Don’t forget to put another return statement after the loop so that the function also returns the correct value when it reaches the end of the array.

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.