composition.js

// a simple function to compose a function
// DO NOT USE THIS IN PRODUCTION 
// you can skip the reading of this function if you want
var compose = function(){
  var fns = Array.prototype.slice.call(arguments)
  return function composed(){
    var thisCall = this
    var args = Array.prototype.slice.call(arguments)
    fns.reverse().forEach(function(fn){
      if (Object.prototype.toString.call(args) !== '[object Array]') args = [args]
      args = fn.apply(thisCall, args)
    })
    return args
  }
}
// take a list of Strings and return a list of up case strings
var toUpperCase = function(list) {
  return list.map(function(x){return x.toUpperCase()})  
}
// take a list of Strings and return a list of length
var length = function(list) {
   return list.map(function(x){return x.length})
}
// take a list of Integer and return the sum
var sum = function(list) {
  var sum = 0
  list.forEach(function(x){sum += x})
  return sum
}
// chain all these functions and produce a new
var sumCharsOnList = compose(sum, length, toUpperCase)
sumCharsOnList(["NX", "PS4", "XboxOne"])

// brief explanation of what happens here
var sumCharsOnList = compose(sum, lenght, toUpperCase)
sumCharsOnList(["NX", "PS4", "XboxOne"])

// --> first the list ["NX", "PS4", "XboxOne"] is passed to the function toUpperCase
// --> toUpperCase returns another list and it's the input for the lenght function
// --> the lenght function return a list of integers which will become the input for sum function
// --> sum function will reduce the list summing all the integers and returning the sum