javascript - How to keep Duplicates of an Array

Question

In Javascript, I'm trying to only keep Duplicates in an Array.

For Example my initial Array is

[1,1,2,3,3,3,3,4,5,5]

the Result should be

[1,3,5]

I've tried working with .indexOf() and $.inArray(), but can't figure it out. I know how to remove Duplicates, but to keep them is quite difficult.

Answer 1

You could filter by checking if the item is the first one and if the last index is not the actual index.

var array = [1, 1, 2, 3, 3, 3, 3, 4, 5, 5],
    result = array.filter((a, i, aa) => aa.indexOf(a) === i && aa.lastIndexOf(a) !== i);

console.log(result);