compose.js

export function compose(...functions) {
	if (functions.length === 0) {
		throw new Error("No functions to compose!");
	}
	return (v) => {
		return functions.reduceRight((value, newFunction) => newFunction(value), v);
	};
}

compose.test.js

import { compose } from "./compose";

test("fails without fns", () => {

	expect(() => compose()).toThrowError("No functions to compose!");
});

test("single fn compose behaves like the same fn", () => {
	const add2 = (n) => n + 2;

	expect(compose(add2)(2)).toEqual(4);
});

test("applies functions from right to left", () => {
	const add2 = (n) => n + 2;
	const divideBy2 = (n) => n / 2;

	expect(compose(divideBy2, add2)(4)).toEqual(3);
});