A function that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flatten.js

// recursively flatten arbitrarily nested array(s) of integers
const flatten = (arr) => {
  // Reduce nested array(s) into a flat array (starting with empty array)
  return arr.reduce((prev, cur) => {
    if (Array.isArray(cur)) {
      // if it's an array, concat its flattened result
      return prev.concat(flatten(cur))
    }
    if (Number.isInteger(cur)) {
      // if it's an integer, concat the value
      return prev.concat(cur)
    }
    // throw if it's not an array or integer
    throw new TypeError('each value of the array must be either an integer or another array')
  }, [])
}

export default flatten

flatten.spec.js

import { expect } from 'chai'
import { describe, it } from 'mocha'
import flatten from './flatten'

describe('flatten', () => {
  it('should flatten deeply nested arrays', () => {
    expect(flatten([[1,2,[3]],4])).to.deep.equal([1, 2, 3, 4])
    expect(flatten([1, [2, 7], 3, [4, 5, 6, [1, 2, [3, [4]]]]])).to.deep.equal([1, 2, 7, 3, 4, 5, 6, 1, 2, 3, 4])
  })
  it('should not change flat arrays', () => {
    expect(flatten([1, 2, 3, 4])).to.deep.equal([1, 2, 3, 4])
    expect(flatten([4])).to.deep.equal([4])
    expect(flatten([])).to.deep.equal([])
  })
  it('should throw if any array value is not an integer or another array', () => {
    expect(() => flatten(['not an integer'])).to.throw(TypeError)
  })
})