citrusbyte problem

flattenArray.js

// Implementation:
const callIfArray = (arr, cb) => Array.isArray(arr) ? cb(arr) : arr;
const flattenRec = (arr) => arr.reduce(
  (acc, curr) => acc.concat(callIfArray(curr, flattenRec)),
  []
);
const flattenArray = (arr) => callIfArray(arr, flattenRec);

// Testing: 
const tests = [
  {
    input: undefined,
    expected: undefined,
  },
  {
    input: [],
    expected: [],
  },
  {
    input: '',
    expected: '',
  },
  {
    input: [[1,2,[3]],4],
    expected: [ 1, 2, 3, 4 ],
  },
  {
    input: [[1,2,[3, [[1,[[[]],[]],2,[3]],[[1,2,[3]],4], [[1,2,[3]],4]]]],4],
    expected: [ 1, 2, 3, 1, 2, 3, 1, 2, 3, 4, 1, 2, 3, 4, 4 ],
  },
  {
    input: [[[1,2,[3]],4], [[1,2,[3]],], [[1,2,[3]],4]],
    expected: [ 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4 ],
  },
]

tests.forEach(({ input, expected }, index) => {
  const actual = flattenArray(input);
  if (JSON.stringify(expected) === JSON.stringify(actual)) {
    console.log('PASSED!')
  } else {
    console.error('Test ', index, ' has failed. Expected:\n', expected, '\nbut got:\n', actual);
  }
})