Flattens an array up to the specified depth.

Use recursion, decrementing depth by 1 for each level of depth.
Use Array..reduce() and Array..concat() to merge elements or arrays.
Base case, for depth equal to 1 stops recursion.
Omit the second argument, depth to flatten only to a depth of 1 (single flatten).

function flatten(arr, depth = 1) {
  return arr.reduce((newArr, curEle) => {
    if (depth > 1 && Array.isArray(curEle)) {
      return newArr.concat(flatten(curEle, depth - 1));
    } else {
      return newArr.concat(curEle);
    }
  }, []);
}
console.log("flatten([1, [2], 3, 4], 3): ",
  flatten([1, [2], 3, 4],
  3));

flatten([1, "a", ["hello", ["world", "1"]]])
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4,
  5], 6], 7],
  8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]