How to Flat nested arrays on Javascript

Flat.js

"use strict";

/**
 * Flats an array of arrays
 * @param {Array} element 
 */
const Flat = (element) => {
  if (!Array.isArray(element))
    return element
  
  return element.reduce((result, curr) => (
    result.concat(Flat(curr))
  ), [])
}

// Tests for Flat function

describe('Flat for int tests', function () {
  it('can flat an empty array', function () {
    const input = []
    const result = Flat(input)
    expect(result).toEqual([])
  });

  it('can flat an array', function () {
    const input = [1]
    const result = Flat(input)
    expect(result).toEqual([1])
  });

  it('can flat an array of arrays', function () {
    const input = [1, 2, 3, [4, 5]];
    const result = Flat(input);
    expect(result).toEqual([1, 2, 3, 4, 5])
  });

  it('can flat an array of arrays of arrays', function () {
    const input = [1, 2, 3, [4, 5], [6, [7, 8], 9]]
    const result = Flat(input)
    expect(result).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9])
  });
});

describe('Flat for string tests', function () {  
  it('can flat an array', function () {
    const input = ['string']
    const result = Flat(input)
    expect(result).toEqual(['string'])
  });

  it('can flat an array of arrays', function () {
    const input = ['hello', ['im', 'flat']]
    const result = Flat(input)
    expect(result).toEqual(['hello', 'im', 'flat'])
  });

  it('can flat an array of arrays of arrays', function () {
    const input = ['hello', ['im', ['flat']]]
    const result = Flat(input)
    expect(result).toEqual(['hello', 'im', 'flat'])
  });
});