About one in every 2,000 people has a condition known as synesthesia, a neurological phenomenon in which one sensory input bleeds into another. The most common form involves assigning colors with letters. However, there is an even rarer form where sentences and paragraphs are associated with tastes and feelings.

For these people, they don't read word by word, sentence by sentence. They look at the whole page/document/program and get a sense for how it tastes—not in the mouth but in the mind. Then they put the parts of the text together like the pieces of a puzzle.

This is what it is like to write fully declarative code: code that describes the relationships between values that tells the machine what we want it to compute. The parts of the program are not instructions in line-by-line order. Synesthetics may be able to do it naturally, but with a little practice anyone can learn how to put the relational puzzle pieces together.

In this chapter, we looked at several mathematical concepts that apply to functional programming and how they allow us to build relationships between data. Next, we'll explore recursion and other advanced topics in JavaScript.