flatten.js

#!/usr/bin/env node

//
//
// Does not use ES6 Spread operator or any external dependency.

// flatten an array of arbitrarily nested arrays into a flat array.
//
// USAGE
//
//  flatten([[1, 2, [3]], 4])  // -> [1, 2, 3, 4]
//
function flatten(array) {
  return _flatten([], array)
}

// I'm a rescurive function.
function _flatten(result, array) {
  array.map(value => {
    if (isArray(value)) {
      _flatten(result, value)
    } else {
      result.push(value)
    }
  })
  return result
}

// A simple test helper, will throw an error if does not match.
function test(name, input, expect) {
  var result = flatten(input)
  if (!equalArray(result, expect)) {
    throw new Error(`${name}: Expected ${JSON.stringify(expect)}, got ${JSON.stringify(result)}`)
  }
}

//
// Helpers
//

// Test is the obj is an Array.
function isArray(obj) {
  return Object.prototype.toString.call(obj) === "[object Array]"
}

// A simple equal for array, not a deep equal
function equalArray(array, other) {
  if (array.length !== other.length) {
    return false
  }
  for (let i=0; i<array.length; i++) {
    if (array[i] !== other[i])
      return false
  }
  return true
}

//
// Main
//

test("empty array", [], [])
test("simple array", [1, 2], [1, 2])
test("one nested array", [1, [2], 3], [1, 2, 3])
test("two nested arrays", [[1, 2, [3]], 4], [1, 2, 3, 4])