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;

With lodash you can solve this with filter and countBy for complexity of O(n):

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

const counts = _.countBy(data, 'id')
console.log(_.filter(data, x => counts[x.id] > 1))

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.10/lodash.min.js"></script>

You could do the same with ES6 like so:

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

const countBy = (d, id) => d.reduce((r,{id},i,a) => (r[id] = a.filter(x => x.id == id).length, r),{})
const counts = countBy(data, 'id')
console.log(data.filter(x => [x.id] > 1))

You can use an array to store unique elements and use filter on values to only return duplicates.

const unique = []

const duplicates = values.filter(o => {

   if(unique.find(i => i.id === o.id && i.name === o.name)) {
     return true
   }

   unique.push(o)
   return false;
})

With lodash you can use _.groupBy() to group elements by their id. Than _.filter() out groups that have less than two members, and _.flatten() the results:

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

const result = _.flow([
  arr => _.groupBy(arr, 'id'), // group elements by id
  g => _.filter(g, o => o.length > 1), // remove groups that have less than two members
  _.flatten // flatten the results to a single array
])(values);

console.log(result);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

An alternative based in @ggorlen solution with new Map() as accumulator (for better performance) and without unary operator ++ (not advised by default in projects with ESLint).

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.set(e.id, (a.get(e.id) ?? 0) + 1);
  return a;
}, new Map());

console.log(values.filter(e => lookup.get(e.id) > 1));

Try this

function checkDuplicateInObject(propertyName, inputArray) {
  var seenDuplicate = false,
  testObject = {};

  inputArray.map(function(item) {
  var itemPropertyName = item[propertyName];    
  if (itemPropertyName in testObject) {
  testObject[itemPropertyName].duplicate = true;
  item.duplicate = true;
  seenDuplicate = true;
 }
 else {
   testObject[itemPropertyName] = item;
   delete item.duplicate;
 }
});

 return seenDuplicate;
}

referred from : http://www.competa.com/blog/lets-find-duplicate-property-values-in-an-array-of-objects-in-javascript/