Function composition allows us to build up powerful functions from smaller, more focused functions. In this lesson we'll demystify how function composition works by building our own compose
and composeAll
functions.
// __test__ import {add, inc, dbl, addInc, addIncDbl} from '../function/custom-compose'; describe('basic fns', () => {
"use strict";
test('add', () => {
const res = add(1,2);
const expected = 3;
expect(res).toBe(expected);
}); test('inc', () => {
const res = inc(2);
const expected = 3;
expect(res).toBe(expected);
}); test('dbl', () => {
const res = dbl(2);
const expected = 4;
expect(res).toBe(expected);
});
}); describe('compose', () => {
"use strict";
test('add then inc', () => {
const res = addInc(4, 2);
const expected = 7;
expect(res).toBe(expected);
});
}); describe('composeAll', () => {
"use strict";
test('add, inc then dbl', () => {
const res = addIncDbl(2, 3);
const expected = 12;
expect(res).toBe(expected);
}) ;
});
/*
* Utils
* */
const compose = (f, g) => (...args) => f(g(...args)); const composeAll = (...fns) => fns.reduce(compose);
/*
* Libs
* */
export const add = (a, b) => a + b; export const inc = (a) => a + 1; export const dbl = (a) => a * 2; export const addInc = compose(inc, add); export const addIncDbl = composeAll(dbl, inc, add);