Flattens nested arrays of numbers

flatten.test.js

/**
Flattens nested arrays of numbers

@param input (nested) array
@return deeply flattened array
*/
function flatten(input) {
  function recursive(arr) {
    if (Array.isArray(arr)) {
      return arr.reduce((r, v) => r.concat(recursive(v)), [])
    }
    if (typeof arr === 'number') {
      return [arr]
    }
    throw new Error('Flatten expects deep array of numbers')
  }

  if (!Array.isArray(input)) {
    throw new Error('Flatten input must be array')
  }
  return recursive(input)
}

describe('flatten', () => {
  const expectToThrow = run => {
    expect(run).toThrow()
  }

  it('rejects undefined', () => {
    expectToThrow(() => flatten(undefined))
  })

  it('flatten nested array', () => {
    const result = flatten([[1, 2, [3]], 4])
    expect(result).toEqual([1, 2, 3, 4])
  })

  it('rejects non-numbers', () => {
    expectToThrow(() => flatten([1, ['a']]))
  })
})