Solution 3 in Sorted Union is confusing

JavaScript
1

Tell us what’s happening:
Solution 3 could be a little bit confusing: filter doesn’t filter the first subarray so the “unique” criteria applied by this function is not consistent with “a function that takes two or more arrays and returns a new array of unique values”. For example:


function uniteUnique(arr) {
var newArr;
var args = Array.prototype.slice.call(arguments);
newArr = args.reduce(function(arrA, arrB) {
  return arrA.concat(
    arrB.filter(function(i) {
      return arrA.indexOf(i) === -1;
    })
  );
});

return newArr;
}

uniteUnique([1, 1, 1], [5, 2, 1, 4], [2, 1]); //-> [ 1, 1, 1, 5, 2, 4 ]

Other solutions produce bullet proof results.

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Challenge: Sorted Union

Link to the challenge: