flatten.js

const flatten = (input) => input.reduce((acc, item) => {
  const flattenedItem = item instanceof Array
    ? flatten(item)
    : item;
  
  return acc.concat(flattenedItem);
}, []);

flatten.test.ts

describe('flatten', () => {
  describe('when array input is not nested', () => {
    it('should return original array', () => {
      const actual = flatten([1,2,3,4]);
      const expected = [1,2,3,4];
      expect(actual).toEqual(expected);
    });
  });

  describe('when array input is one-level nested', () => {
    it('should return flattened array', () => {
      const actual = flatten([1,2,3,[4]]);
      const expected = [1,2,3,4];
      expect(actual).toEqual(expected);
    });
  });

  describe('when array input is more than one-level nested', () => {
    it('should return flattened array', () => {
      const actual = flatten([[[0]], 1,2,[3],[4, [5]]]);
      const expected = [1,2,3,4,5];
      expect(actual).toEqual(expected);
    });
  });
});