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);

1const findDuplicates = (arr) => {
2  let sorted_arr = arr.slice().sort(); // You can define the comparing function here. 
3  // JS by default uses a crappy string compare.
4  // (we use slice to clone the array so the
5  // original array won't be modified)
6  let results = [];
7  for (let i = 0; i < sorted_arr.length - 1; i++) {
8    if (sorted_arr[i + 1] == sorted_arr[i]) {
9      results.push(sorted_arr[i]);
10    }
11  }
12  return results;
13}
14
15let duplicatedArray = [9, 4, 111, 2, 3, 9, 4, 5, 7];
16console.log(`The duplicates in ${duplicatedArray} are ${findDuplicates(duplicatedArray)}`);

1function doubleValues(array) {
2  var newArray = [];
3  array.forEach(function (el) { newArray.push(el, el); });    
4  return newArray;
5}
6
7console.log(doubleValues([1,2,3]));