Function composition is the final key to understanding functional programming. Function composition takes many of the concepts learned in this chapter and nicely wraps them in the core of functional programming. The widely used definition of function composition is that function composition is a mathematical concept that allows you to combine multiple functions to create a new function. This definition tells us what function composition is, but doesn't really give us any sense of how to compose functions or why we need to use it.

As we know from the definition, function composition is the act of combining functions to create a new one. What does this mean exactly? In mathematics, we often see functions composed like so: f(g(x)). If this is not familiar to you, in the expression f(g(x)), we pass the variable x into the function g and then pass the result of g(x) into the function f. The expression f(g(x)) is evaluated from the inside out, from right to left, in the order...