symdiff_es6.js

function sym(args) {

  //solution with ES6
  //instead of doing manual work, make set operations
  
  //combine values
  var union = (set1, set2) => new Set([...set1, ...set2]);
  //Common values, set1 values, if set2 has them, keep them
  var intersection = (set1, set2) => new Set( [...set1].filter(value => set2.has(value)) );
  //keep values that are found only in set1
  var difference = (set1, set2) => new Set( [...set1].filter(value => !set2.has(value)) );
  
  //first make the arguments into an array
  //map each array inside into a set, now we have a list of sets
  //take union of A & B minus their intersection
  var result = [...arguments]
               .map(arr => new Set(arr))
               .reduce( (A, B) => difference( union(A, B), intersection(A, B) ));
  
  return [...result];
}

References

• http://www.2ality.com/2015/01/es6-set-operations.html
• https://github.com/FreeCodeCamp/wiki/blob/master/Algorithm-Symmetric-Difference.md
• https://en.wikipedia.org/wiki/Symmetric_difference