I found a couple of similar questions, but they were mostly outdated. Here's the problem: create a flat array from the values in several arrays contained in several nested objects.

const data = [
  { id: 1, items: ['one', 'two', 'three'] },
  { id: 2, items: ['two', 'one', 'four'] },

Expected result const result = ['one', 'two', 'three', 'four']

My current solution:

function() {
  const result = []
  data.forEach(item => data.items.forEach(item => result.push(item)))

  return _.uniq(result)

lodash is allowed

Any suggestion is more than welcome


2 Answers 2


Although at the time of writing, flatMap is not in every browser (according to MDN not in Edge and Samsung Internet);

data.flatMap(obj => obj.items).filter((e, i, ary) => ary.indexOf(e) == i)

Although, you could check the uniqueness from the result array in your code, and push only if not in there, to save one loop over the array.

Another option would be a reducer like

uniqItems = (acc, {items}) => acc.concat(items.filter(item => acc.indexOf(item) < 0))

to data.reduce(uniqItems, []).


Use a Set, it will store unique items for you.

You can convert the set to an array with [...set.values()];

Or as a set is iterateable there is no need to convert it to an array until needed if at all.

const data = [{items: ['one', 'two', 'three'] },{items: ['two', 'one', 'four'] }];

// Quickest solution
function getUniqueA(arr) {
    const unique = new Set();
    for (const {items} of arr) {
        for (const item of items) { unique.add(item) }
    return [...unique.values()];

// Smallest solution
const getUniqueB = arr => [...(new Set(arr.map(i => i.items).flat())).values()];
// Returns the set
const getUniqueC = arr => new Set(arr.map(i => i.items).flat());
const resA = getUniqueA(data);
const resB = getUniqueB(data);
const resC = getUniqueC(data);

logArray("Result A: ", resA);
logArray("Result B: ", resB);
logArray("Result C: ", ...resC);

function logArray(t, ...a) { console.log(t + `[${a}]`) }


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