How to flatten an array using recursion in Javascript

Problem statement: There is an array containing elements as an array and those array elements can contain other elements as an array and so on. Now we have to convert this input array to a flat array that contains elements but not an array of elements.

Example:
Input: [1,2,3,[4,5,[6,7]],8,[9,[10,11]]]
Output: [1,2,3,4,5,6,7,8,9,10,11]

Solution:

function flatten(arr) {
  let flattenArr = [];
  arr.forEach(el => {
    if(Array.isArray(el)){
      const result = flatten(el);
      result.forEach(el => flattenArr.push(el));
    } else {
      flattenArr.push(el);
    }
  });
  return flattenArr;
}

const input = [1,2,3,[4,5,[6,7]],8,[9,[10,11]]];
const output = flatten(input);

console.log(output);

We have used recursion to solve this problem

Algorithm steps:

• First, we iterate through the given array.
• Then check each element:
  • if it is not an array then push the elements in an updated array.
  • if it is an array then again call the same function flatten() i.e. recursion. Then we will combine our updated array and return values of flatten() using the spread operator in ES6. This will keep flatting the updated array.

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Comments

[Peter Vivo]

Good solution, but maybe use .flat

[1,2,3,[4,5,[6,7]],8,[9,[10,11]]].flat() // [1,2,3,4,5,[6,7],8,9,[10,11]]

flat parameter is recursion level:

[1,2,3,[4,5,[6,7]],8,[9,[10,11]]].flat(2) // [1,2,3,4,5,6,7,8,9,10,11]

[Rohit Kumawat]

Thanks for reading the article and for this solution 🙌. Really appreciate it.
But yeah I covered this solution because it covers recursion as well.

[Vesa Piittinen]

The good old super compatible way to do it:

function flatten(array) {
    // only mutate what you own, thus create a new copy
    array = array.slice(0);
    for (i = 0; i < array.length; i++) {
        // Array.isArray(array[i])
        if (Object.prototype.toString.call(array[i]) === '[object Array]') {
            // replaces single item with all items from the array
            Array.prototype.splice.apply(array, [i, 1].concat(array[i--]));
        }
    }
    return array;
}

This should work with pretty much any JavaScript engine released within last 20 years, and does it without recursion.

But as others are saying you should just use .flat() these days instead of reinventing the wheel.

[Rohit Kumawat]

Thanks for reading the article and for this solution 🙌. Really appreciate it.
But yeah I covered this solution because it covers recursion as well.

[Pacharapol Withayasakpunt]

• Why would you have unstructured data required to be flattened like that in the first place?
• Classical solution for this would be using .reduce or an equivalent.

const flatten = (arr) => arr.reduce((prev, c) => [...prev, ...c], [])

Then, for a more complex version.

const flatten = (arr) => arr.reduce((prev, c) => [...prev, Array.isArray(c) ? ...flatten(c) : c], [])

[Ashutosh Biswas]

In the recursive version, JavaScript produces SyntaxError(It would be nice if it wouldn't) cause ternary operator does not allow spreading like this. We can solve it using spread operators if we really want to, but I've found concat requires a little less typing:

const flatten = (arr) => arr.reduce((prev, c) => prev.concat(Array.isArray(c) ? flatten(c) : c), [])

[Rohit Kumawat]

Thanks for reading the post and for another solution. I just wanted to cover use cases of recursion.

[RWRFyZA]

Hello ^^. Nice use of recursion !

For the sake of discussion, here is another solution using built-in methods of Array :

const input = [1,2,3,[4,5,[6,7]],8,[9,[10,11]]]

let output = input.reduce((flattenArr, el)=>{
  if(Array.isArray(el)){
    flattenArr = flattenArr.concat(el.flat(2))
  }else{
    flattenArr.push(el)
  }
  return flattenArr
}, [])

console.log(output)

One could argue that if your input's data are nested to a higher level, you will have to adapt the flat() method accordingly, which is not an issue in your code.
But won't you be suppose to know the shape of the data you're working with ^^ ?

Thank you for your time and showing us that recursion is not terrifying !

[Rohit Kumawat]

Thanks for reading the article and for this solution 🙌. Really appreciate it. But yeah I covered this solution because it covers recursion as well.

[Jim Kernicky]

Yes you can use the flat() method but I was looking for a recursion solution - thanks a lot!