Functional programming in JavaScript and TypeScript

Work in progress.

See https://www.manning.com/books/functional-programming-in-javascript.

Expressiveness

FP can lead to more concise and expressive code, which can be easier to read and maintain once the paradigm is fully understood.

In JavaScript, which supports functional programming concepts, combining map, reduce, and filter functions showcases the expressiveness of the language. These functions can operate on arrays to transform, filter, and accumulate values in a clear declarative style.

Here’s an example that demonstrates their combined use:

// A sample array of objects representing people with age and name
const people = [
  { name: "Alice", age: 25 },
  { name: "Bob", age: 20 },
  { name: "Carol", age: 30 },
  { name: "Dave", age: 35 },
  { name: "Eve", age: 40 },
];

// We want to calculate the average age of people older than 25
const averageAge = people
  .filter((person) => person.age > 25) // Filter out people aged 25 or below
  .map((person) => person.age) // Transform the filtered list into a list of ages
  .reduce((acc, age, _, array) => {
    // Reduce to accumulate ages and compute average
    acc += age;
    return array.length - 1 === index ? acc / array.length : acc;
  }, 0);

console.log(averageAge); // Expected output: Average age of people older than 25

Let’s go step by step:

Filter: The filter function is used first to obtain only the people whose ages are above 25.
Map: After filtering, we use the map function on the resulting array to extract just the ages of the remaining people.
Reduce: Finally, the reduce function accumulates the ages and computes the average by dividing the accumulated sum by the length of the filtered array.

This chain of filter, map, and reduce demonstrates how you can clearly and succinctly express a complex data transformation without having to write imperative loops and temporary variables. It shows what needs to be done, abstracting away the details of how it should be implemented under the hood.