Given a multi-dimensional array arr and a depth n, return a flattened version of that array.

A multi-dimensional array is a recursive data structure that contains integers or other multi-dimensional arrays.

A flattened array is a version of that array with some or all of the sub-arrays removed and replaced with the actual elements in that sub-array. This flattening operation should only be done if the current depth of nesting is less than n. The depth of the elements in the first array are considered to be 0.

Please solve it without the built-in Array.flat method.

// 當 current depth of array < n 時，就會被 flattened
// 要再多判斷不是 array 的不用再被 flatten
var flat = function (arr, n) {
  const newArray = [];

  function flatten(ele, depth) {
    if (depth < n && Array.isArray(ele)) { 
      for (let i = 0; i < ele.length; i++) {
        flatten(ele[i], depth + 1);
      }
    } else {
      newArray.push(ele);
    }
  }
  for (let i = 0; i < arr.length; i++) {
    flatten(arr[i], 0);
  }

  return newArray;
};

Recursion function

recursion 是指一個函式會在自己的定義中呼叫自己，直到滿足某個基礎條件（base condition）時才會停止。這個基礎條件的存在是為了終止遞迴。

舉例：列出所有物件裡的 children name

function printChildrenRecursive(t) {
  if (t.children.length === 0) { // base condition
    return;
  }
  t.children.forEach(c => {
    console.log(c.name);
    printChildrenRecursive(c);
  });
}

const tree = {
  name: 'John',
  children: [
    { name: 'Jim', children: [] },
    {
      name: 'Kyle',
      children: [
        { name: 'Denny', children: [] },
        { name: 'Nancy', children: [] },
      ],
    },
  ],
};