Pairwise an array in Javascript

function pairwise(arr) {
    return arr.map(function (item, index, _arr) {
        return _arr.map(function (_item) {
            if (item != _item) {
                return [item, _item];
            }
        }).filter(Boolean);
    });
}

an example:

pairwise(["Cat","Dog","Bird"]);
# outputs: 
array(3): {
   [0]:  array(2): {
      [0]:  array(2): {
         [0]:  string(3): "Cat"
         [1]:  string(3): "Dog"
      }
      [1]:  array(2): {
         [0]:  string(3): "Cat"
         [1]:  string(4): "Bird"
      }
   }
   [1]:  array(2): {
      [0]:  array(2): {
         [0]:  string(3): "Dog"
         [1]:  string(3): "Cat"
      }
      [1]:  array(2): {
         [0]:  string(3): "Dog"
         [1]:  string(4): "Bird"
      }
   }
   [2]:  array(2): {
      [0]:  array(2): {
         [0]:  string(4): "Bird"
         [1]:  string(3): "Cat"
      }
      [1]:  array(2): {
         [0]:  string(4): "Bird"
         [1]:  string(3): "Dog"
      }
   }
}