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

here is mine, es6-like, deps-free, answer. with filter instead of reducer

// this checks if elements of one list contains elements of second list 
// example code
[0,1,2,3,8,9].filter(item => [3,4,5,6,7].indexOf(item) > -1)

// function
const contains = (listA, listB) => listA.filter(item => listB.indexOf(item) > -1) 
contains([0,1,2,3], [1,2,3,4]) // => [1, 2, 3]

// only for bool
const hasDuplicates = (listA, listB) => !!contains(listA, listB).length

---

edit: hmm my bad is: I've read q as general question but this is strictly for lodash, however my point is - you don't need lodash in here :)

Answer 2

Another way is to group by unique items, and return the group keys that have more than 1 item

_([1, 1, 2, 2, 3]).groupBy().pickBy(x => x.length > 1).keys().value()

Answer 3

Well you can use this piece of code which is much faster as it has a complexity of O(n) and this doesn't use Lodash.

[1, 1, 2, 2, 3]
.reduce((agg,col) => {
  agg.filter[col] = agg.filter[col]? agg.dup.push(col): 2;
  return agg
 },
 {filter:{},dup:[]})
.dup;

//result:[1,2]

Answer 4

You can use this:

_.filter(arr, (val, i, iteratee) => _.includes(iteratee, val, i + 1));

Note that if a number appears more than two times in your array you can always use _.uniq.

Answer 5

No need to use lodash, you can use following code:

function getDuplicates(array, key) {
  return array.filter(e1=>{
    if(array.filter(e2=>{
      return e1[key] === e2[key];
    }).length > 1) {
      return e1;
    }
  })
}

Answer 6

var array = [1, 1, 2, 2, 3];
var groupped = _.groupBy(array, function (n) {return n});
var result = _.uniq(_.flatten(_.filter(groupped, function (n) {return n.length > 1})));

This works for unsorted arrays as well.