lodash: Get duplicate values from an array

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

[1, 1, 2, 2, 3]

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

[1, 2]

Does lodash support this? I want to do it in the shortest way possible.

8 Answers
8

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.

_.uniq

const duplicates = _.filter(array, (value, index, iteratee) => _.includes(iteratee, value, index + 1))

_.find

_.includes

_.filter(array, (value, index, iteratee) => return _.find(iteratee, value, index + 1) )

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()


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.

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]


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

countBy()

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

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]


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 :)

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);


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