What is the JS pattern to get unique elements from nested array?

Question

I have the following result from MongoDB.aggregate:

[{
   _id: ObjectId(1), 
   _author: ObjectId(2),
   comments: [
      {
         _author: ObjectId(2),
         text: '...'
      },
      {
         _author: ObjectId(3),
         text: '...1'
      },
      {
         _author: ObjectId(3),
         text: '...2'
      }...
   ]
}...]

I need to get all unique authors _author field from all elemnts (including nested):

var uniqAuthors = magicFunction(result) // [ObjectId(2), ObjectId(3)] ;

What is the best and compact way to make it with pure JS?

Answer 1

Array.prototype.reduce can help you:

var unique = result[0].comments.reduce(function(uniqueAuthors, comment) {
  if (uniqueAuthors.indexOf(comment._author) === -1) {
     uniqueAuthors.push(comment._author);
  }
  return uniqueAuthors;
}, []);
//Verify the author from document
if (unique.indexOf(result[0]._author) === -1) {
   uniqueAuthors.push(result[0]._author);
}