A composition is a very important concept of functional programming; it is how we create a higher-order function by consuming and combining simpler functions.

Let's just use the sum function that we defined previously. We can split the sum function into the following composition:

const sum = a => b => a + b
sum(2)(5)

Any function can be transformed into a composable function via the concept of currying. Explaining these fundamental functional concepts is beyond the scope of this book, and we suggest that you get familiar with functional terms as much as possible, in order to get a full understanding of Redux and React. We will consume a composition function from Redux, called compose, in upcoming chapters.