Very simple solution for symmetric difference between sets with functional set abstraction. (freecodecamp.org coding interview execise: https://www.freecodecamp.org/learn/coding-interview-prep/algorithms/find-the-symmetric-difference)

find-the-symmetric-difference.js

function exists(s, x) {
  for (let i = 0; i < s.length; i++) {
    if (s[i] === x) 
      return true
  }
  return false
}

function insert(s, x) {
  if (!exists(s, x)) 
    return [...s, x]
  return s
}

function union(s1, s2) {
  let res = [...s1]

  for (let i = 0; i < s2.length; i++) {
    res = insert(res, s2[i])
  }

  return res
}

function intersect(s1, s2) {
  let res = []

  for (let i = 0; i < s1.length; i++) {
    if (exists(s2, s1[i]))
      res = insert(res, s1[i])
  }

  return res
}

function diff(s1, s2) {
  let res = []

  for (let i = 0; i < s1.length; i++) {
    if (!exists(s2, s1[i])) {
      res = insert(res, s1[i])
    }
  }

  return res
}

function sym(s1, s2, ...args) {
  const symDiff = diff(union(s1, s2), intersect(s1, s2));

  if (args.length == 0) 
    return symDiff
  
  return sym(symDiff, ...args)
}

sym([1, 2, 3], [5, 2, 1, 4]);