Return objects with duplicate properties to a new array [duplicate]

Question

I've seen a lot of examples that show how to filter/remove duplicate values from array, but I want to get the duplicates and use them in a new array. So for

array = [
    { id: '1', description: 'foo'},
    { id: '2', description: 'bar'},
    { id: '3', description: 'bloop'},
    { id: '1', description: 'bleep'}
]

I want a new array returned that only contains items with matching Id's like so

[
  { id: '1', description: 'foo'},
  { id: '1', description: 'bleep'}
]

I feel like this should be easier than I'm making it

Edit: The suggestions do not do what I'm asking for. They are either creating lookup tables or removing the items from the original array.

@ChristopherMellor you claim that your question is substantially different from the suggested duplicate. (i.e. the accepted answer just creates a "lookup table.") However, the answer you have accepted here is functionally identical to the answer accepted over there. — kdbanman
– kdbanman, Commented Jan 27, 2021 at 23:41

GouriSankar · Accepted Answer · 2021-01-27 19:33:53Z

1

array = [
    { id: '1', description: 'foo'},
    { id: '2', description: 'bar'},
    { id: '3', description: 'bloop'},
    { id: '1', description: 'bleep'}
]

const duplicates = array.reduce((a, e) => {
  a[e.id] = ++a[e.id] || 0;
  return a;
}, {});

console.log(array.filter(e => duplicates[e.id]));

answered Jan 27, 2021 at 19:33

GouriSankar

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