Beginner-friendly example of refactoring JavaScript to functional programming style

combineKeyedArrays.js

// See this blog post for full explanation:
// https://rebootjeff.github.io/blog/2015/12/11/example-refactoring-to-functional-js-combine-keyed-lists/

// Dependencies
var _ = require('lodash');
var R = require('ramda');

// Example input
var usersBySocialNetwork = {
  twitter: [
    { name: '@RebootJeff' },
    { name: '@doitwithalambda' },
    null
  ],
  facebook: [
    null,
    { name: 'Kevin' },
    { name: 'Bianca' },
  ]
};

// Expected output
var users = [
  { name: '@RebootJeff' },
  { name: '@doitwithalambda' },
  { name: 'Kevin' },
  { name: 'Bianca' }
];

// Original Solution
function combineKeyedArrays_v1(keyedArrays){
  var flattened = [];

  // produce a flat Array from an Object with values that are arrays
  _.each(keyedArrays, function(array){
    flattened = flattened.concat(array);
  });

  // only return the truthy elements of flat Array
  return _.filter(flattened, function(element) {
    return Boolean(element);
  });
}

// Refactored using Lodash's Chain
function combineKeyedArrays_v2(keyedArrays){
  return _.chain(keyedArrays)
    .reduce(concatArray, [])
    .filter(Boolean)
    .value();
}

function concatArray(arr, val) {
  return arr.concat(val);
}

// Refactored using Lodash's function composition
var combineKeyedArrays_v3 = _.flow(
  _.values,
  _.flatten,
  _.compact
);

// Refactored using Ramda's function composition and currying
var combineKeyedArrays_v4 = R.pipe(
  R.values,
  R.unnest,
  R.filter(Boolean)
);

// Possible type signatures for documentation purposes:
// Object<Array> --> Array (more old-fashioned)
// ...or...maybe...
// {k: [v]} --> [v] (similar to Ramda docs)