1let strArray = [ "q", "w", "w", "w", "e", "i", "u", "r"];
2let findDuplicates = arr => arr.filter((item, index) => arr.indexOf(item) != index)
3
4console.log(findDuplicates(strArray)) // All duplicates
5console.log([...new Set(findDuplicates(strArray))]) // Unique duplicates

1var array = [1, 2, 2, 3, 3, 4, 5, 6, 2, 3, 7, 8, 5, 22, 1, 2, 511, 12, 50, 22];
2
3console.log([...new Set(
4  array.filter((value, index, self) => self.indexOf(value) !== index))]
5);

1// JavaScript - finds if there is duplicate in an array. 
2// Returns True or False.
3
4const isThereADuplicate = function(arrayOfNumbers) {
5    // Create an empty associative array or hash. 
6    // This is preferred,
7    let counts = {};
8    // // but this also works. Comment in below and comment out above if you want to try.
9    // let counts = [];
10
11    for(var i = 0; i <= arrayOfNumbers.length; i++) {
12        // As the arrayOfNumbers is being iterated through,
13        // the counts hash is being populated.
14        // Each value in the array becomes a key in the hash. 
15        // The value assignment of 1, is there to complete the hash structure.
16        // Once the key exists, meaning there is a duplicate, return true.
17        // If there are no duplicates, the if block completes and returns false.
18        if(counts[arrayOfNumbers[i]] === undefined) {
19            counts[arrayOfNumbers[i]] = 1;
20        } else {
21            return true;
22        }
23    }
24    return false;
25}

1var input = [1, 2, 3, 1, 3, 1];
2
3var duplicates = input.reduce(function(acc, el, i, arr) {
4  if (arr.indexOf(el) !== i && acc.indexOf(el) < 0) acc.push(el); return acc;
5}, []);
6
7document.write(duplicates); // = 1,3 (actual array == [1, 3])

1function findUniq(arr) {
2  return arr.find(n => arr.indexOf(n) === arr.lastIndexOf(n));
3}
4
5console.log(findUniq([ 0, 1, 0 ]))
6console.log(findUniq([ 1, 1, 1, 2, 1, 1 ]))
7console.log(findUniq([ 3, 10, 3, 3, 3 ]))
8console.log(findUniq([ 7, 7, 7, 20, 7, 7, 7 ]))

1var names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']
2
3var uniq = names
4  .map((name) => {
5    return {
6      count: 1,
7      name: name
8    }
9  })
10  .reduce((a, b) => {
11    a[b.name] = (a[b.name] || 0) + b.count
12    return a
13  }, {})
14
15var duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)
16
17console.log(duplicates) // [ 'Nancy' ]