Flatten nested arrays using recursion in JavaScript

Question

I'm trying to flatten nested arrays while preserving the order, e.g. [[1, 2], 3, [4, [[5]]]] should be converted to [1, 2, 3, 4, 5].

I'm trying to use recursion in order to do so, but the code below does not work and I don't understand why. I know there are other methods to do it, but I'd like to know what's wrong with this.

function flatten (arr) {
  var newArr = [];
  for (var i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      flatten(arr);
    } else {
      newArr.push(arr[i]);
    }
  }
  return newArr;
}

flatten([[1, 2], 3, [4, [[5]]]]);

Thanks

Sorry, it's one of the freecodecamp challenges and I changed the name for something more descriptive, but I forgot that one. That's not the issue though. — user1576121, Jun 01 '15 at 20:18
See my recursive version of array flattening in [this stackoverflow page](https://stackoverflow.com/questions/39287320/js-array-concatenation-for-results-of-recursive-flattening/67145361#67145361) — Sandro Rosa, Apr 18 '21 at 08:20

score 14 · Accepted Answer · answered Jun 01 '15 at 20:20

14

When calling flatten recursively, you need to pass arr[i] to it and then concat the result with newArr. So replace this line:

flatten(arr);

with:

newArr = newArr.concat(flatten(arr[i]));

answered Jun 01 '15 at 20:20

OrenD

1,751
13
12

score 5 · Answer 2 · answered Jun 13 '17 at 16:09

5

Here's a common pattern that I regularly use for flattening nested arrays, and which I find a bit more clean due to its functional programming nature:

var flatten = (arrayOfArrays) =>
    arrayOfArrays.reduce((flattened, item) =>
        flattened.concat(Array.isArray(item) ? flatten(item) : [item]), []);

Or for those who like a shorter, less readable version for code golf or so:

var flatten=a=>a.reduce((f,i)=>f.concat(Array.isArray(i)?flatten(i):[i]),[]);

answered Jun 13 '17 at 16:09

Maurice Makaay

53
1
2

Line `Array.isArray(item) ? flatten(item) : [item]`: `[item]` should be `item` => `Array.isArray(item) ? flatten(item) : item` – tanguy_k Feb 24 '18 at 10:08

score 1 · Answer 3 · answered Jun 01 '15 at 20:21

Here is some working code

function flatten (arr) {
  var newArr = [];
  for (var i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      var temp = flatten(arr[i]);
      temp.forEach(function(value){ newArr.push(value); })
    } else {
      newArr.push(arr[i]);
    }
  }
  return newArr;
}

score 1 · Answer 4 · answered Jun 01 '15 at 20:24

1

Havent tested it, bit this part

if (Array.isArray(arr[i])) {
      flatten(arr);
    } else {

Seems to be intended as

if (Array.isArray(arr[i])) {
  flatten(arr[i]);
} else {

answered Jun 01 '15 at 20:24

edmarisov

138
5

Where's the difference? – Bergi Jun 01 '15 at 21:04
@Bergi difference is in passing a [i] instead of a. But one thing missing here -as OrenD mentioned is concatenation of results – edmarisov Jun 02 '15 at 05:42
Ah, you should've stated that explicitly in your answer. I only spotted the changed indentation. – Bergi Jun 02 '15 at 12:06

score 0 · Answer 5 · answered Apr 16 '21 at 20:24

This solution is without using any Array methods

//1 >> loop inside parent
//2 >> if the element is not an array then push it in array that has to be returned or else loop inside it

const arr = [[0, 1], [2, 3], [4, 5, [6, 7, [8, [9, 10]]]]]

function toFlatArray(arr) {
  let flatArray = [];
  function rec(a) {
    if (!Array.isArray(a)) {
      flatArray.push(a);
    } else {
      for (let i = 0; i < a.length; i++) {
        rec(a[i]);
      }
    }
  }
  rec(arr);
  return flatArray;
}



const flatArray = toFlatArray(arr);

console.log("flatArray", flatArray);

_..This solution is without using any Array methods..._ Please explain what is the benefit of your answer compared to others — B001ᛦ, Apr 16 '21 at 21:53

score 0 · Answer 6 · answered Jun 27 '21 at 18:39

Easy to understand solution:

var flatten = function(array) {
    let flatArray = [];
  
  array.forEach(element => {
    if (Array.isArray(element)) {
      let inner = flatten(element);
      flatArray.push(...inner)
    } else {
        flatArray.push(element);
    }
  })
  
  return flatArray;
};

score 0 · Answer 7 · answered Aug 19 '21 at 17:20

Recursive Solution!


let newArray = [];
function arrayFlat(array){
    for(let i = 0; i< array.length; i++){
        if(!array[i].length) { newArray.push(array[i])};
        arrayFlat(array[i]);
    }
    console.log(newArray);
}

arrayFlat(arrayData);

// Array to be flattened.
arrayData = [1,2,[3,4],[[5,6],7]];

Flatten nested arrays using recursion in JavaScript

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