How to search for users by both first and last name with MongoDB?

I see couple of mistakes in your code causing undesired result.

1. Aggregation pipeline accepts array of aggregation framework operations. In your case, you are missing [] operator. It should be like

   User.aggregate([{$project...},{$match...}])

2. In $match stage you are using regex, if you are using /../ style of regex, you don't need to wrap it around string quotes. It should be /bob j/i

Here is finished example:

User.aggregate([
  {$project: { "name" : { $concat : [ "$firstName", " ", "$lastName" ] } }},
  {$match: {"name": {$regex: /bob j/i}}}
]).exec(function(err, result){
  console.log(result);
});

You should see [ { _id: 574c3e20be214bd4078a9149, name: 'Bob Jerry' } ] on screen.