Bonfire: Diff Two Arrays - I had stuck here and finally figured out, pick up the right solution!

diff.js

function diff(a1, a2) {
 var a = {}, diff = [];
  for(var i = 0; i < a1.length; i++) {
    a[a1[ i ]] = a1[ i ];
  }

  for(var j=0; j < a2.length; j++) {
    if(a[a2[ j ]]) {
      delete a[a2[ j ]];
    } else {
      a[a2[ j ]] = a2[ j ];
    }
  }

  return Object.keys(a).map(function(prop) {
    return a[ prop ];
  });
}

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