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Flatten an array

Array.flat

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Question

Write a method which will flatten any array

inputArray = [1, 2, [3, 4], 5, [[[6, 7], 8, [[[[9]]]]]]]
function flatten(inputArray) => output [1, 2, 3, 4, 5, 6, 7, 8, 9]

Thought process Pseudocode

  1. We attach the method to the array prototype, the input is the this array itself,
  2. We create a result Flatarray, and iterate through the input array.
  3. For each value check if it's an array itself, if so recursively call the same function
  4. if the element is not an array, push it into the result array.

Solution

Array.prototype.flatten = function () {
  let flatArray = []
  for (let index = 0; index < this.length; index++) {
    const element = this[index];
    if (Array.isArray(element)) {
      flatArray = flatArray.concat(this.flatten.call(element))
    } else {
      flatArray.push(element)
    }
  }
  return flatArray;
}
// function flatten
//const nestedArr = [[1], [[1, 4, [5, 3]], [1, 2, 3, 4]]] => [1, 1, 4, 5, 3, 1, 2, 3, 4]
const nestedArr = [[1], [[1, 4, [5, 3]], [1, 2, 3, [3, 4, [2, [22, [3, 4, 5, 6, 5, [2]]]]], 4]]]
//const nestedArr = [1, 2, 3, 4, [1]]
const flat = nestedArr.flatten();
console.log(flat);

Caveat

As we're creating a new array at every recursive call, this won't be a very handy method if you have large arrays in that case we can simply use array.push