JS Bin// source https://jsbin.com/zalogohafu

functionComposition.js

// based on https://www.youtube.com/watch?v=mth5WpEc4Qs

"use strict";

var add1 = function add1(num) {
  return num + 1;
};
var doubleIt = function doubleIt(num) {
  return num * 2;
};

// equivalent to (acc, cur) => {return cur(acc)}
var apply = function apply(fn1, fn2) {
  return fn2(fn1);
};

// we use currying so that the function applies the array of fns, but waits on val
var compose = function compose(arr) {
  return function (val) {
    return arr.reduce(apply, val);
  };
};

// and this is where we test this out
var add1AndDoubleIt = compose([add1, doubleIt]);
console.log(add1AndDoubleIt(2));

functionCompositionES6.js

// based on https://www.youtube.com/watch?v=mth5WpEc4Qs


const add1 = num => num + 1
const doubleIt = num => num * 2

// equivalent to (acc, cur) => {return cur(acc)}
const apply = (fn1, fn2) => fn2(fn1)

// we use currying so that the function applies the array of fns, but waits on val
const compose = arr => val => arr.reduce(apply, val)

// and this is where we test this out
const add1AndDoubleIt = compose([add1, doubleIt])
console.log(add1AndDoubleIt(2))

jsbin.zalogohafu.js

// based on https://www.youtube.com/watch?v=mth5WpEc4Qs

"use strict";

var add1 = function add1(num) {
  return num + 1;
};
var doubleIt = function doubleIt(num) {
  return num * 2;
};

// equivalent to (acc, cur) => {return cur(acc)}
var apply = function apply(fn1, fn2) {
  return fn2(fn1);
};

// we use currying so that the function applies the array of fns, but waits on val
var compose = function compose(arr) {
  return function (val) {
    return arr.reduce(apply, val);
  };
};

// and this is where we test this out
var add1AndDoubleIt = compose([add1, doubleIt]);
console.log(add1AndDoubleIt(2));