find duplicate string in array javascript

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);

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' ]

1let a = [1, 2, 3, 4, 2, 2, 4, 1, 5, 6]
2let b = [...new Set(a.sort().filter((o, i) => o !== undefined && a[i + 1] !== undefined && o === a[i + 1]))]
3
4// b is now [1, 2, 4]