flattenArray

// const nested = [1, [2, [3, [4, [5]]]]];

// // Flatten one level (the default).
// nested.flat(); // [1, 2, [3, [4, [5]]]]

// // Flatten a specific number of levels.
// nested.flat(2); // [1, 2, 3, [4, [5]]]

// // Flatten all levels by passing Infinity.
// nested.flat(Infinity); // [1, 2, 3, 4, 5]

export default function flatten(arr) {
  const flattenArr = (arr, resultArr) => {
    const result = resultArr || [];
    for (let i = 0; i < arr.length; i++) {
      if (Array.isArray(arr[i])) {
        flattenArr(arr[i], result);
      } else {
        result.push(arr[i]);
      }
    }
    return result;
  }
  return flattenArr(arr, []);
}


// Test case
```
import flatten from './flatten';

describe('flatten array', () => {
  test('empty array', () => {
    expect(flatten([])).toEqual([]);
  });

  test('nested array', () => {
    expect(flatten([1, [2]])).toEqual([1, 2]);
  });

  test('multiple levels of nesting', () => {
    expect(flatten([1, [2, [3]]])).toEqual([1, 2, 3]);
  });
});
```