lodash: Get duplicate values from an array

Question

Say I have an array like this: [1, 1, 2, 2, 3]

I want to get the duplicates which are in this case: [1, 2]

Does lodash support thi

Answer 1

Another way, but using filters and ecmaScript 2015 (ES6)

var array = [1, 1, 2, 2, 3];

_.filter(array, v => 
  _.filter(array, v1 => v1 === v).length > 1);

//→ [1, 1, 2, 2]

Answer 2

Hope below solution helps you and it will be useful in all conditions

  hasDataExist(listObj, key, value): boolean {
    return _.find(listObj, function(o) { return _.get(o, key) == value }) != undefined;
  }



  let duplcateIndex = this.service.hasDataExist(this.list, 'xyz', value);

Answer 3

Here is another concise solution:

let data = [1, 1, 2, 2, 3]

let result = _.uniq(_.filter(data, (v, i, a) => a.indexOf(v) !== i))

console.log(result)

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

_.uniq takes care of the dubs which _.filter comes back with.

Same with ES6 and Set:

let data = [1, 1, 2, 2, 3]

let result = new Set(data.filter((v, i, a) => a.indexOf(v) !== i))

console.log(Array.from(result))

Answer 4

How about using countBy() followed by reduce()?

const items = [1,1,2,3,3,3,4,5,6,7,7];

const dup = _(items)
    .countBy()
    .reduce((acc, val, key) => val > 1 ? acc.concat(key) : acc, [])
    .map(_.toNumber)

console.log(dup);
// [1, 3, 7]

http://jsbin.com/panama/edit?js,console

Answer 5

pure js solution

export function hasDuplicates(array) {
  return new Set(array).size !== array.length
}

Answer 6

You can make use of a counter object. This will have each number as key and total number of occurrence as their value. You can use filter to get the numbers when the counter for the number becomes 2

const array = [1, 1, 2, 2, 3],
      counter = {};
      
const duplicates = array.filter(n => (counter[n] = counter[n] + 1 || 1) === 2)

console.log(duplicates)