Find duplicate items in arrays using ES6

findDuplicateItemsArrays.js

// ES6 

// Count duplicate items
const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

const count = names => 
  names.reduce((a, b) => 
    Object.assign(a, {[b]: (a[b] || 0) + 1}), {})

const duplicates = dict => 
  Object.keys(dict).filter((a) => dict[a] > 1)

console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
console.log(duplicates(count(names))) // [ 'Nancy' ]

// Return array with duplicates

// prep
  const arr = Array.from('Learn more javascript dude');
  const counter = (prev, next) => Object.assign(prev, { [next] : (prev[next] || 0) + 1 });
  const singles = function(key){ return this[key] === 1 };
  const multiples = function(key){ return this[key] > 1 };

  // work
  const counted = arr.reduce(counter, {});
  const filtered = Object.keys(counted).filter(multiples.bind(counted));

  //[ "e", "a", "r", " ", "d" ]
  console.log(filtered);

const duplicates = (arr) => {
let seen = new Set();
let store = new Array();
arr.filter(item => seen.size === seen.add(item).size && !store.includes(item) && store.push(item))
return store;
}