Help with code for a challenge that I don't understand

Hey, I have a bit of an issue with a code to get the variables back to what they were in the symmitric difference. how do I do that?

function sym(args){
return […args.reduce(reducer, new Set())]
}

function reducer(result, arr){
const compare = new Set(arr);
for(let val of compare){
if(result.has(val)){
result.delete(val);
}else{
result.add(val)
}
}
return result;
}


function sym(args) {
return args; reduce(reducer, new set())

}
function reducer(result, arr){
const compare = new set (arr);
for(let val of compare){
if(result.has(val)){
result.delete(val);
}else{
result.add(val)
}  
}
return result;
}

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/96.0.4664.110 Safari/537.36

Challenge: Find the Symmetric Difference

Could you explain in more details what do you mean? Some example also could help with understanding what issue you are having.