flatten.js

export function flatten(inputArr) { 
  return inputArr.reduce((outputArr, input) => {
    if (Array.isArray(input)) {
      return outputArr.concat(flatten(input))
    }

    outputArr.push(input)
    return outputArr
  }, [])
}

flatten.test.js

import { flatten } from './flatten'

test('it can flatten two dimentional arrays', () => {
  expect(flatten([[1],[2]])).toEqual([1,2])
})

test('it can flatten more than two dimentions', () => {
  expect(flatten([1,2,[3,4],[[5,6],[7]],[8]])).toEqual([1,2,3,4,5,6,7,8])
})

test('it can flatten mixed value types', () => {
  expect(flatten(['1',2,[3,{}],[[5,6],[[]]],[.5]])).toEqual(['1',2,3,{},5,6,.5])
})

Hi Richard,
your code looks good, thanks for your submission.

Just one question: what happens if I pass something that's not an array to the flatten function?
Could you refactor it so that it throws an error?

Thanks
Stefano