Create flat array

function equals(arr1, arr2) {
  return JSON.stringify(arr1) === JSON.stringify(arr2);
}

function flatten(arr) {
  return arr.reduce((result, toFlatten) => {
    return result.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}

const tests = [
  {
    it: [[1,2,[3]],4],
    should: [1, 2, 3, 4]
  },
  {
    it: [[1,0,2,[3]],4],
    should: [1, 0, 2, 3, 4]
  },
  {
    it: [[[1,[[0]],2,[3]],4]],
    should: [1, 0, 2, 3, 4]
  },
  {
    it: [1],
    should: [1]
  },
  {
    it: [],
    should: []
  },
]

for (let { it, should } of tests) {
  console.log(equals(flatten(it), should))
}