freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/steamroller/index.md

---
title: Steamroller
---

# Steamroller

---
## Problem Explanation

This problem seems simple but you need to make sure to flatten any array, regardless of the level which is what adds a bit of difficulty to the problem.

#### Relevant Links

*   <a href='http://forum.freecodecamp.com/t/javascript-array-isarray/14284' target='_blank' rel='nofollow'>Array.isArray()</a>


---
## Hints

### Hint 1

You need to check if an element is an array or not.

### Hint 2

If you are dealing with an array, then you need flatten it by getting the value inside of the array. This means if you have `[[4]]` then instead of returning `[4]` you need to return `4`. If you get `[[[4]]]` then the same, you want the `4`. You can access it with `arr[index1][index2]` to go a level deeper.

### Hint 3

You will definitely need recursion or another way to go beyond two level arrays to make the code flexible and not hard-coded to the answers needed. Have fun!


---
## Solutions

<details><summary>Solution 1 (Click to Show/Hide)</summary>

```javascript
function steamrollArray(arr) {
  var flattenedArray = [];

  // Create function that adds an element if it is not an array.
  // If it is an array, then loops through it and uses recursion on that array.
  var flatten = function(arg) {
    if (!Array.isArray(arg)) {
      flattenedArray.push(arg);
    } else {
      for (var a in arg) {
        flatten(arg[a]);
      }
    }
  };

  // Call the function for each element in the array
  arr.forEach(flatten);
  return flattenedArray;
}

// test here
steamrollArray([1, [2], [3, [[4]]]]);
```


#### Code Explanation

*   Create a new variable to keep flattened arrays.
*   Create a function that will add non-array elements to the new variable, and for the ones that are array, loop through them to get the element.
*   It does that by using recursion, if the element is an array then call the function again with a layer of array deeper to check if it is an array or not. If it is not then push that non-array element to the variable that gets returned. Otherwise, keep going deeper.
*   Invoke the function, the first time you will always pass it an array, so it always falls into the isArray branch
*   Return the flattened array.

#### Relevant Links

*   <a href='http://forum.freecodecamp.com/t/javascript-array-prototype-push/14298' target='_blank' rel='nofollow'>Array.push()</a>
*   <a href='http://forum.freecodecamp.com/t/javascript-array-prototype-foreach/14290' target='_blank' rel='nofollow'>Array.forEach()</a>
</details>


<details><summary>Solution 2 (Click to Show/Hide)</summary>

```javascript
function steamrollArray(arr) {
  let flat = [].concat(...arr);
  return flat.some(Array.isArray) ? steamrollArray(flat) : flat;
}

steamrollArray([1, [2], [3, [[4]]]]);
```


#### Code Explanation

*   Use spread operator to concatenate each element of `arr` with an empty array
*   Use `Array.some()` method to find out if the new array contains an array still
*   If it does, use recursion to call `steamrollArray` again, passing in the new array to repeat the process on the arrays that were deeply nested
*   If it does not, return the flattened array

#### Relevant Links

*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some' target='_blank' rel='nofollow'>Array.some</a>
*   <a href='http://forum.freecodecamp.com/t/javascript-array-prototype-concat/14286' target='_blank' rel='nofollow'>Array.concat</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator' target='_blank' rel='nofollow'>Spread operator</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator' target='_blank' rel='nofollow'>Ternary Operator (`condition ? a : b`)</a>

</details>

<details><summary>Solution 3 (Click to Show/Hide)</summary>

```javascript
function steamrollArray(arr) {
  while (arr.some(element => Array.isArray(element))) {
    arr = arr.flat();
  }
  return arr;
}

steamrollArray([1, [2], [3, [[4]]]]);
```


#### Code Explanation

*   Use `Array.some()` method to find out if the new array contains an array still, if it does flatten the array
*   Repeats until there are no more arrays inside arr.

#### Relevant Links

*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some' target='_blank' rel='nofollow'>Array.some</a>
*   <a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat' target='_blank' rel='nofollow'>Array.flat</a>

</details>

<details><summary>Solution 4 (Click to Show/Hide)</summary>

```javascript
function steamrollArray(arr) {
  return arr
    .toString()
    .replace(",,", ",") // "1,2,,3" => "1,2,3"
    .split(",") // ['1','2','3']
    .map(function(v) {
      if (v == "[object Object]") {
        // bring back empty objects
        return {};
      } else if (isNaN(v)) {
        // if not a number (string)
        return v;
      } else {
        return parseInt(v); // if a number in a string, convert it
      }
    });
}
```


#### Code Explanation

*   First we turn the array to a string, which will give us a string of numbers separated by a comma, double comma if there was an empty array and literal object text if there was an object, which we can fix later in our if statement.
*   We replace the double comma with one, then split it back into an array.
*   map through the array and fix object values and convert string numbers to regular numbers.

</details>

```javascript
const steamrollArray = arr => arr.flat(Infinity);
```

#### Code Explanation

*   Use `Array.flat()` to flatten an array with `Infinity` as a parameter for the depth.