Implement a deepFlattenArray function

Apr 01, 2024

Deep Flattening refers to the process of transforming a nested array into a single-level array by flattening all nested arrays within it. We already have the Array.prototype.flat() method available to use, but in this case we are recreating it.

Variation: Basic implementation

1. Basic implementation

Flatten the array to for any level of nesting.

function deepFlattenArray(arr) {
  const result = [];

  function deepFlattenArrayImpl(input) {
    input.forEach((item) => {
      if (Array.isArray(item)) {
        deepFlattenArrayImpl(item);
      } else {
        result.push(item);
      }
    });
  }

  deepFlattenArrayImpl(arr);

  return result;
}

// Usage
const inputArr = [1, [2, [3, [4, [{ name: 'John Doe' }, null], 0], -1]]];
const output = deepFlattenArray(inputArr);

/**
 * Yields:
 * [
    1,
    2,
    3,
    4,
    {
        "name": "John Doe"
    },
    null,
    0,
    -1
]
 */

console.log(output);

Variation: With Depth

2. With Depth implementation

In the previous case, we simply flattened the array to for any level of nesting. This variation will have a depth field upto which the flattening needs to be performed.

As most of the implementation remains exactly same, I will highlight only the newer changes made.

function deepFlattenArray(arr, depth = 1) {
  const result = [];

  function deepFlattenArrayImpl(input, depth) {
    input.forEach((item) => {
      // if flattening is needed and depth is not exhausted
      if (Array.isArray(item) && depth > 0) {
        deepFlattenArrayImpl(item, depth - 1);
      } else {
        result.push(item);
      }
    });
  }

  deepFlattenArrayImpl(arr, depth);

  return result;
}

// Usage
const inputArr = [1, [2, [3, [4, [{ name: 'John Doe' }, null], 0], -1]]];
const output = deepFlattenArray(inputArr, 1);

/**
 * Yields:
 [
    1,
    2,
    [
        3,
        [
            4,
            [
                {
                    "name": "John Doe"
                },
                null
            ],
            0
        ],
        -1
    ]
]
 */

console.log(output);

Variation: As Polyfill

3. As polyfill implementation

As Array.prototype.flat() is already present to be used on arrays, in an interview setting one might also expect to implement a polyfill for this. The solution to that is also staright forward and only requires an understanding of this keyword and prototype property on constructor functions.

if (!Array.prototype.flat) {
  Array.prototype.flat = function (depth = 1) {
    const arr = this;
    // original array should not be mutated
    return deepFlattenArray(arr, this);
  };
}

Implement a deepFlattenArray function

1. Basic implementation

2. With Depth implementation

3. As polyfill implementation

Further Reading