Find the Symmetric Difference Please help

Tell us what’s happening:

Your code so far


function sym(args) {
  return args;
}

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

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.80 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/coding-interview-prep/algorithms/find-the-symmetric-difference

I can’t understand this challenge anybody can help me!

What specifically do you not understand? Be as detailed as possible, so we can try to explain what you are missing.