index.js

module.exports = function flatten(nestedArrays) {
  if (!Array.isArray(nestedArrays))
    throw new Error("flatten() called with non-array");
  function* f(nestedArrays) {
    for (let element of nestedArrays)
      if (Array.isArray(element)) yield* f(element);
      else yield element;
  }
  return Array.from(f(nestedArrays));
}

index.test.js

//tests written with Jest

const flatten = require('./')

describe('Flatten', () => {
  const nonArray = {a: {b:{}}}
  const stringArray = ['a', 'b', ['c', 'd']]
  const intArray = [1, 2, [3, 4]]
  const deepIntArray = [1,2,[1,2,[1,2,['a', 'b']]]]

  it('should fail if flatten receives non-array', () => {
    expect(() => flatten(nonArray)).toThrowError('flatten() called with non-array')
  })

  it('should flatten array of strings', () => {
    expect(flatten(stringArray)).toEqual(['a', 'b', 'c', 'd'])
  })

  it('should flatten array of integers', () => {
    expect(flatten(intArray)).toEqual([1, 2, 3, 4])
  })

  it('should flatten array of deep integers', () => {
    expect(flatten(deepIntArray)).toEqual([1, 2, 1, 2, 1, 2, 'a', 'b'])
  })
})