Javascript function to return an array containing the items that occur in one but not both input arrays. Need to refactor this.

diff.js

/**
 * diff two arrays - return the elements that occur in one but not both input arrays
 * note to self: This works but there has to be a better (more performant) solution
 *               Revisit this and look into removing items from both arrays as they're discovered
 **/
function diff(arr1, arr2) {
  // get the items from arr1 that aren't in arr2
  var filteredArr1 = arr1.filter( function( el ) {
    return arr2.indexOf( el ) < 0;
  } );
  
  // now get the items from arr2 that aren't in arr1
  var filteredArr2 = arr2.filter( function( el ) {
    return arr1.indexOf( el ) < 0;
  } );  
  
  // group them up and return them
  return filteredArr1.concat(filteredArr2);
}

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