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]));

You can simplify this if your browser supports Object.groupBy:

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

const lookup = Object.groupBy(values, e => e.id);
console.log(values.filter(e => lookup[e.id].length > 1));