Flatten a nested array into a single level array

flatten.js

const assert = require('assert')
/**
 * Returns a nested array into a flatten single level array
 * @param  {Array} input a multiple level array
 * @return {Array}  an one level array of all the elements inside
 */
function flatten (input) {
  return input.reduce((ret, el) => {
    if (Array.isArray(el)) {
      ret = ret.concat(flatten(el))
      return ret
    }
    ret.push(el)
    return ret
  }, [])
}

const expected = [1, 2, 3, 4, 5]
const normal = [1, 2, 3, 4, 5]
assert.deepEqual(expected, flatten(normal))
const oneLevel = [1, 2, 3, 4, [5]]
assert.deepEqual(expected, flatten(oneLevel))
const multipleLEvels = [[[[[[[[1]]]]]]], 2, 3, 4, 5]
assert.deepEqual(expected, flatten(multipleLEvels))
const severalArrays = [[1], [2, 3], [4, 5]]
assert.deepEqual(expected, flatten(severalArrays))
const fullyNested = [[1, 2, 3, 4, 5]]
assert.deepEqual(expected, flatten(fullyNested))
const severalLevels = [[[1], [2, [[3]]], [[[4]], 5]]]
assert.deepEqual(expected, flatten(severalLevels))