Learning Recursion in JavaScript Part 3 - Flattening Arrays

Last reviewed on April 5, 2019

Ah, recursion, one of those intimidating programming topics that make many developers' heads spin 🤯 . These types of problems can be difficult to conceptualize, especially when you're under pressure during an interview. I know I've felt that before! Well, that's why I am writing this blog post series on recursion, to help you and me learn it better. As I've spent more time learning about it, I've come to find it a fascinating topic and appreciate how elegantly you can use it to solve certain types of problems.

Here are the other posts in this series if you haven't checked those out yet:

  1. The Obligatory Factorial Function
  2. Sum an Array of Numbers 3 Ways
  3. Flattening Arrays
  4. Palindromes
  5. Revisiting the Factorial Function with Tail Recursion

For this third post in this series on recursion, we're going to look at writing a function to flatten a nested array with an arbitrary depth. For example, let's say we have the following array:

let array = [[1, 2], 3, [4, [5]]];

Let's write a function to flatten it as such:

flat(array); // [1, 2, 3, 4, 5];

So before we get into writing this function, it is worth noting that as of ES2019, there is a new Array.prototype.flat() method for flattening arrays. I wrote a post about it called Flattening Arrays in JavaScript with flat() and flatMap() if you'd like to learn more about it. The flat() function that we will write in this post will behave a little bit differently, as it will completely flatten an array with any depth.

Our flat() Function

So to start off, how would you go about flattening an array that is only one level deep? For example:

let array = [[1, 2], 3, [4, 5]];

One way you could do it is using Array.prototype.reduce():

function flat(array) {
  return array.reduce((acc, item) => {
    return acc.concat(item);
  }, []);

let array = [[1, 2], 3, [4, 5]];
flat(array); // [1, 2, 3, 4, 5]

So how can we expand this implementation to flatten for an arbitrary depth? Let's look at the following array:

let array = [
    /* might have nesting */

The values in this array are in two possible states that we care about. That is, a value can either contain an array or not. If a value is an array, we want to keep flattening. If the value isn't an array, then there is nothing to flatten. This condition is our base case! The base case can be expressed in code as such:

Array.isArray(item) ? flat(item) : item;

Here we are using the ternary operator. If the item is an array, call the flat() function again. Otherwise, don't, which will stop the recursion. When we add this to our flat() function, it looks like this:

function flat(array) {
  return array.reduce((acc, item) => {
    return acc.concat(Array.isArray(item) ? flat(item) : item);
  }, []);

Flattening Arrays with an Arbitrary Depth

Our function works great, but let's take it a step further and allow for a specified depth to be passed in as an argument, similar to Array.prototype.flat(depth).

Here is the implementation and an example of how to use it with and without a depth argument:

function flat(array, depth, currentDepth = 0) {
  if (currentDepth === depth) {
    return array;

  return array.reduce((acc, item) => {
    return acc.concat(
      Array.isArray(item) ? flat(item, depth, currentDepth + 1) : item
  }, []);

let array = [[1, 2], 3, [4, [5]]];

flat(array, 1); // [1, 2, 3, 4, [5]]
flat(array); // [1, 2, 3, 4, 5]

We've added 2 more parameters: depth and currentDepth which has a default value of 0. The currentDepth parameter is used to keep track of how deep we are recursing. Every time we recurse, we increment that number. Then, at the top of flat(), we do a check to see if currentDepth equals depth. If those values are equal, we stop the recursion. Otherwise, we keep going. If a depth argument isn't passed into flat(), depth will be undefined. Thus, currentDepth, which starts off at 0, will never equal undefined, and our function will flatten the array for however deep it is.


Data that has some arbitrary level of nesting can often times be elegantly solved with recursion, such as the infinitely nested array in this post. When learning recursion, I have found that if often helps to look at the problem as if it weren't infinitely nested. For example, we first looked at the array in this post and solved it as if it had a depth of 1. Then, we determined what the condition was to recurse and when to stop (the base case).

Stay tuned for the next post in my series on recursion, as we'll continue to look at more examples and facets of recursion.