// Created on savesnippets.com · https://savesnippets.com/E0ckac5UnLuEd0
const flatDeep = (arr, depth = Infinity) => arr.flat(depth);

// Manual fallback for environments without Array.flat:
function flatDeepManual(arr, depth = Infinity) {
  return depth > 0
    ? arr.reduce(
        (acc, val) =>
          acc.concat(Array.isArray(val) ? flatDeepManual(val, depth - 1) : val),
        []
      )
    : arr.slice();
}

// Usage
const nested = [1, [2, [3, [4, [5]]]]];
console.log(flatDeep(nested));        // [1, 2, 3, 4, 5]
console.log(flatDeep(nested, 2));     // [1, 2, 3, [4, [5]]]