Iterative solution for flattening n-th nested arrays in Javascript

Question

Can anyone show me an iterative solution for the following problem? I solved it recursively but struggled with an iterative solution. (Facebook Technical Interv

Answer 1

Here are two approaches, recursive and iterative and their comparison to Array.flat. Maybe it'll help someone

const arrayToFlatten = [[1], [2, [3]], null, [[{}]], undefined];

// takes an array and flattens it recursively, default depth is 1 (just like Array.flat())
function flattenRecursive(arr, depth = 1) {
  let myArray = [];

  if (depth === 0){ // if you've reached the depth don't continue
    myArray = arr;
  } else if(!Array.isArray(arr)) { // add item to array if not an array
    myArray.push(arr);
  } else { // flatten each item in the array then concatenate
    arr.forEach(item => {
      const someNewArray = flattenRecursive(item, depth - 1);
      myArray = myArray.concat(someNewArray);
    });
  }
  
  return myArray;
}

// takes an array and flattens it using a loop, default depth is 1 (just like Array.flat())
function flattenIterative(arr, depth = 1) {
  let result = arr;

  // if an element is an array
  while(result.some(Array.isArray) && depth) {
    // flatten the array by one level by concating an empty array and result using apply
    result = [].concat.apply([], result);
    depth--; // track depth
  }

  return result;
}

console.log(arrayToFlatten.flat(2)); // ES^
console.log(flattenRecursive(arrayToFlatten, 2));
console.log(flattenIterative(arrayToFlatten, 2));