[Javascript] Understand Function Composition By Building Compose and ComposeAll Utility Functions

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);