Function Composition

Function composition is another concept that leaks over from mathematics.

Given two functions, a and b, compose returns a new function that applies a to the output of b, which is then applied to a given set of parameters.

Function composition is a way to create a complex function from a set of smaller ones.

This will mean you might end up with a bunch of simple functions that do one thing well. Functions with a single purpose are better at encapsulating their functionality and therefore help with separation of concerns.

Composing functions ties in with currying and the partial application of functions since currying/partial application is a technique that allows you to have specialized versions of generic functions, like so:

const sum = x => y => x + y;
const multiply = x => y => x * y;
const compose = (f, g) => x => f(g(x));
const add1 = sum(1);
const add2 = sum(2);
const double = multiply(2);

To explain the following code...