The first set of operations we are going to consider, work on an array, and process it in the base of a function to produce some results. There are several possible results: a single value, with the .reduce() operations; a new array, with .map(); or just about any kind of result, with .forEach().
Note
If you Google around, you will find some articles that declare that these functions are not efficient, because a loop done by hand can be faster. This, while possibly true, is practically irrelevant. Unless your code really suffers from speed problems, and you are able to measure that the slowness derives from the usage of these higher-order functions, trying to avoid them, with longer code, and more probability of bugs simply doesn't make much sense.
Let's start by considering the preceding list of functions in order, starting by the most general of all--which, as we'll see, can even be used to emulate the rest of the transformations in this chapter!