JS function to flatten an arbitrarily nested array

flatten-array.js

// creates a new array with all sub-array elements concatenated into it recursively
// equivalent to array.prototype.flat with Infinity passed for depth
const flattenArray = (arr) => 
  arr.reduce(
    (result, value) => result.concat(
      Array.isArray(value)
        ? flattenArray(value)
        : value
    ),
    [],
  )

// TESTS

const assert = require('assert')

const testCases = [
  [1, [2, 3, [4], 5]],
  [1, 2, 4],
  [],
  [1],
  [[[[[[7]]]]]],
]

testCases.forEach(arr => {
  const actual = flattenArray(arr)
  const expected = arr.flat(Infinity)
  assert.deepEqual(
    expected,
    actual,
    `
    expected: ${JSON.stringify(expected)}
    actual: ${JSON.stringify(actual)}
    ` 
  )
})