Programmers with the classical software engineering background need to learn more mathematics these days. The world is changing; we have to change, too. This book is dedicated to covering the issues that until recently were not very popular in software engineering: logic, monoids, algebraic structures, categories, and monads. All these topics are explained in the book, with no assumptions about the reader's educational background, with many examples. Most of the examples and explanations use two popular programming languages, JavaScript and Scala. The book has no theorems and almost no proofs. The purpose of the book is to expand the reader's imagination and to open the gates to the beautiful world of mathematics—at the same time keeping in mind the practical usability of its ideas and notions in our daily coding practice.