Array Flatten

flatten.js

const flatten = (source) => {
  const length = source.length
  let i = 0
  let flattened = []

  for (; i < length; i++) {
    // recursive call could reach stack limit
    flattened = flattened.concat(
      !Array.isArray(source[i]) ? source[i] : flatten(source[i])
    )
    // could iterate and expand first item in array until not an array
    // then push onto flattened output
  }
  return flattened
}

console.assert(
  arrayEqual(
    flatten([ 1, 2, 3, 4 ]),
    [ 1, 2, 3, 4 ]
  ),
  `doesn't work with flat arrays`
)
console.assert(
  arrayEqual(
    flatten([ [ 1, 2, [ 3 ] ], 4 ]),
    [ 1, 2, 3, 4 ]
  ),
  `doesnt flatten simple nested arrays`
)

const deeplyNestedArray = Array.from({ length: 10000 }).reduce(acc => [ acc ], 0)
console.assert(
  arrayEqual(
    flatten(deeplyNestedArray),
    [ 0 ]
  ),
  `doesn't flatten deeply nested array`
)

function arrayEqual(arr1, arr2) {
  const length = arr1.length
  let i = 0
  if (length !== arr2.length) return false
  for (; i < length; i++)
    if (arr1[i] !== arr2[i])
      return false
  return true
}