Get list of duplicate objects in an array of objects

Question

I am trying to get duplicate objects within an array of objects. Let's say the object is like below.

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

Duplicate objects should return like below:

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

Answer 1

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]));

Answer 2

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/