Source https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat https://bigfrontend.dev/problem/implement-Array-prototype.flat

function flat(arr, depth = 1) {
  // Recursively flatten for each depth
  if (depth > 0) {
    return arr.reduce((acc, val) => {
      if (Array.isArray(val)) {
        return [...acc, ...flat(val, depth - 1)];
      } else {
        return [...acc, val];
      }
    }, []);
  // Base case if no more depth to flatten, return the new array
  } else {
    return arr.slice();
  }
}