symmetricDifference.js

function sym(...args) {

  function filterRepeat(arr) {
    const filteredList = [];
    
    for (const n of arr) {
      if (!filteredList.includes(n)) {
        filteredList.push(n);
      }
    }
    
    return filteredList;
  }

  while (args.length > 1) {

    const arr1 = filterRepeat(args[0]);
    const arr2 = filterRepeat(args[1]);
    const combined = arr1.concat(arr2);

    const freqObj = {};

    for (const n of combined) {
      const key = String(n);
      if (freqObj[key] === undefined) {
        freqObj[key] = 1;
      } else {
        freqObj[key]++;
      }
    }

    const result = [];

    for (const key in freqObj) {
      if (freqObj[key] == 1) {
        result.push(Number(key));
      }
    }

    args.splice(0, 2, result);

  }

  return args[0];

}

console.log(sym([1, 1, 2, 5], [2, 2, 3, 5], [3, 4, 5, 5])); // [1, 4, 5]