Took the ES5 output of the tuts+ course on functional programming, http://code.tutsplus.com/courses/functional-programming-in-javascript, and slapped some ES6 in there instead.

tutsplus.es6.js

!function(_) {

'use strict';

let beerData = JSON.parse(document.getElementById("beerData").textContent),
allBeers     = beerData.beers,
beerTemplate = document.getElementById("tmpl-beer-groups").textContent,
beerList     = document.getElementById("beerList"),
averageAbv   = document.getElementById("averageAbv"),
filters      = document.getElementById("filters"),
filterLinks  = filters.querySelectorAll("a");

let fp = {
  mean(collection, property) {
    if(property) {
      collection = _.pluck(collection, property);
    }
    return _.reduce(collection, fp.add, 0) / collection.length;
  },
  roundDecimal(number, places) {
    let factor = Math.pow(10, places);
    return Math.round(number * factor) / factor;
  },
  add(a, b) {
    return a + b;
  }
};

function loadBeers(beers) {
  let beerGroups = _.groupBy(beers, (beer) => beer.locale);
  beerList.innerHTML = _.template(beerTemplate)({ beers: beerGroups });
  averageAbv.innerHTML = `Average ABV: ${getAverageAbv(beers)}%`;
}

function setActiveFilter(active) {
  _.forEach(filterLinks, (link) => {
    link.classList.remove('btn-active');
  });
  active.classList.add('btn-active');
}

function makeFilter(collection, property) {
  return (value) => _.filter(collection, (item) => item[property] === value);
}

function getAverageAbv(beers) {
  let mean = fp.mean(beers, 'abv');
  return fp.roundDecimal(mean, 1);
}

let filterByLocale = makeFilter(allBeers, 'locale');
let filterByType   = makeFilter(allBeers, 'type');

loadBeers(allBeers);

filters.addEventListener('click', (e) => {
  e.preventDefault();
  let clicked   = e.target,
  filterName    = clicked.dataset.filter,
  filteredBeers = [];

  setActiveFilter(clicked);

  switch (filterName) {
    case 'all':
      filteredBeers = allBeers;
      break;
    case 'domestic':
      filteredBeers = filterByLocale('domestic');
      break;
    case 'imports':
      filteredBeers = filterByLocale('import');
      break;
    case 'ale':
      filteredBeers = _.filter(allBeers, (beer) => beer.type === 'ale' || beer.type === 'ipa');
      break;
    case 'lager':
      filteredBeers = filterByType('lager');
      break;
    case 'stout':
      filteredBeers = filterByType('stout');
      break;
    case 'surprise':
      filteredBeers = [_.sample(allBeers)];
      break;
  }

  loadBeers(filteredBeers);
});

}(_);