A Javascript function to flatten the provided array elements into a single array

flatten.js

function flatten(input) {
  return input.reduce((acc, current) => {
    return acc.concat(Array.isArray(current) ? flatten(current) : current)
  }, [])
}


/**
 * Tests
 **/
import flatten from "./flatten"

describe("flatten", () => {
  const assertions = [
    { value: [], expected: [] },
    {
      value: [[1, 2, 3], [4, 5, 6], [7, 8, 9]],
      expected: [1, 2, 3, 4, 5, 6, 7, 8, 9]
    },
    { value: [[1, 2, [3]], 4], expected: [1, 2, 3, 4] },
    { value: [[1], [2], [3], [4]], expected: [1, 2, 3, 4] },
    {
      value: [[[[1]], 2, [3]], [4], [5, [6, [7, [[8]]]]]],
      expected: [1, 2, 3, 4, 5, 6, 7, 8]
    }
  ]

  it("works with deeply nested arrays", () => {
    assertions.forEach(({ value, expected }) => {
      expect(flatten(value)).toEqual(expected)
    })
  })

  it("works with falsy values", () => {
    expect(flatten([null, [undefined], [0], [100]])).toEqual([
      null,
      undefined,
      0,
      100
    ])
  })
})