Integer flatten example while taking depth into account (via node version v7.1.0)

array-flatten.js

const flattenInternal = (input, initialDepth, acculamator = [], depth = 0) => {
  if (initialDepth !== undefined && depth > initialDepth) {
    throw new Error(`array has exceeded the target depth of ${initialDepth}.`)
  }

  return input.reduce((acc, value) => {
    if (Array.isArray(value)) {
      if (initialDepth !== undefined) {
        return flattenInternal(value, initialDepth, acc, depth + 1)
      }

      return flattenInternal(value, initialDepth, acc, depth)
    }

    if (!Number.isInteger(value)) {
      throw new Error(`wrong again bra (expected ${value} to be an integer but got ${typeof value}). try again.`)
    }

    return [...acc, value]
  }, acculamator)
}

module.exports = (input, depth = undefined) => {
  if (depth !== undefined && !Number.isInteger(depth)) {
    throw new Error('depth must be an integer.')
  }

  return flattenInternal(input, depth)
}

array-flatten.test.js

const flatten = require('./array-flatten')
const assert = require('assert')

describe('flatten integers', () => {
  it('should throw if array depth is exceeded', () => {
    assert.throws(() => flatten([[1,2,[3]],4], 1), /array has exceeded the target depth of 1./)
  })

  it('should throw if depth is not an integer', () => {
    assert.throws(() => flatten([[1,2,[3]],4], 'not-an-integer'), /depth must be an integer\./)
  })

  it('should throw if depth is null', () => {
    assert.throws(() => flatten([[1,2,[3]],4], null), /depth must be an integer\./)
  })

  it('should not throw if array depth is not exceeded', () => {
    assert.deepEqual(flatten([[1,2,[3]],4], 2), [1,2,3,4])
  })

  it('should throw if any element is not an integer', () => {
    assert.throws(() => flatten([[1,2,[3]],'oo']), /wrong again bra \(expected oo to be an integer but got string\)\. try again\./)
  })

  it('should allow an array of arbitrary depth if no depth is provided', () => {
    assert.deepEqual(flatten([[1,2,[3, [118, [333], 12]]],88]), [1,2,3,118,333,12,88])
  })

  it('should allow an array of arbitrary depth if depth is undefined', () => {
    assert.deepEqual(flatten([[1,2,[3, [118, [333], 12]]],88], undefined), [1,2,3,118,333,12,88])
  })
})