compose.js

// ES6 compose
const compose = (...funcs) => 
  x => funcs.reduceRight((acc, func) => func(acc), x);

// ES5 equivelant
var compose = function(){
  var args = Array.prototype.slice.call(arguments);

  return function(val){
    return args.reduceRight(function(memo, fn){
      return fn(memo);
    }, val);
  };
};

// ES6 version explaination
const compose // define a constant veriable called compose
= (...funcs) =>  // compose is a function that takes an arbitrary number of args and puts them in an array called funcs
x => // compose returns a function 
funcs.reduceRight((acc, func) => func(acc), x) // ...
// when the function returned from compose is invoked with x, it will set x as the initial value to reduceRight.
// reduceRight will iterate right-to-left over the original array of functions. It will apply each function to
// the accumulator, then the result of each function invokation will be set as the new accumulator

// Example
const add1 = a => a + 1; // take a number, add 1 to it, and return
const mult5 = a => 5 * a; // take a number, multiply it by 5, and return

const add1ThenMult5 = compose(mult5, add1);
// add1ThenMult5 = x => [mult5, add1].reduceRight((acc, func) => func(acc), x)
add1ThenMult5(5)
// first we do add1(5) and set the result of that operation (6) as the new accumulator
// then we do mult5(6) and return that value (30) as our result, since we reached the beginning of our array