Get list of duplicate objects in an array of objects

You can use Array#reduce to make a counter lookup table based on the id key, then use Array#filter to remove any items that appeared only once in the lookup table. Time complexity is O(n).

const values = [{id: 10, name: 'someName1'}, {id: 10, name: 'someName2'}, {id: 11, name:'someName3'}, {id: 12, name: 'someName4'}];

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

console.log(values.filter(e => lookup[e.id]));

Let's say you have:

arr = [
    { id:10, name: 'someName1' },
    { id:10, name: 'someName2' },
    { id:11, name: 'someName3' },
    { id:12, name: 'someName4' }
]

So, to get unique items:

unique = arr
     .map(e => e['id'])
     .map((e, i, final) => final.indexOf(e) === i && i)
     .filter(obj=> arr[obj])
     .map(e => arr[e]);

Then, result will be

unique = [
     { id:10, name: 'someName1' },
     { id:11, name: 'someName3' },
     { id:12, name: 'someName4' }
]

And, to get duplicate ids:

duplicateIds = arr
     .map(e => e['id'])
     .map((e, i, final) => final.indexOf(e) !== i && i)
     .filter(obj=> arr[obj])
     .map(e => arr[e]["id"])

List of IDs will be

duplicateIds = [10]

Thus, to get duplicates objects:

duplicate = arr.filter(obj=> dublicateIds.includes(obj.id));

Now you have it:

duplicate = [
    { id:10, name: 'someName1' },
    { id:10, name: 'someName2' }
]

Thanks https://reactgo.com/removeduplicateobjects/

You haven't clarified whether two objects with different ids, but the same "name" count as a duplicate. I will assume those do not count as a duplicate; in other words, only objects with the same id will count as duplicate.

let ids = {};
let dups = [];

values.forEach((val)=> {
  if (ids[val.id]) {
    // we have already found this same id
    dups.push(val)
  } else {
    ids[val.id] = true;
  }
})
return dups;