What does $$this mean in MongoDB Aggregation?

Question

In a Mongo Aggregation example I've encountered the expression $$this, but cannot find a reference to it in the MongoDB documentation (not even in the documentation about aggregation variables)

Here is the sample data:

{ "_id" : 1, "actions" : [ 2, 6, 3, 8, 5, 3 ] }
{ "_id" : 2, "actions" : [ 6, 4, 2, 8, 4, 3 ] }
{ "_id" : 3, "actions" : [ 6, 4, 6, 4, 3 ] }
{ "_id" : 4, "actions" : [ 6, 8, 3 ] }
{ "_id" : 5, "actions" : [ 6, 8 ] }
{ "_id" : 6, "actions" : [ 6, 3, 11, 8, 3 ] }
{ "_id" : 7, "actions" : [ 6, 3, 8 ] }

Here is the code

Here is the code I'm looking at:

db.test.aggregate([
    {$match:{actions:{$all:[6,3,8]}}},
    {$project:{actions638:{$map:{
       input:{$range:[0,{$subtract:[{$size:"$actions"},2]}]},
       in:{$slice:["$actions","$$this",3]}
    }}}}
])

and here is the output

{ "_id" : 1, "actions638" : [ [ 2, 6, 3 ], [ 6, 3, 8 ], [ 3, 8, 5 ], [ 8, 5, 3 ] ] }
{ "_id" : 2, "actions638" : [ [ 6, 4, 2 ], [ 4, 2, 8 ], [ 2, 8, 4 ], [ 8, 4, 3 ] ] }
{ "_id" : 4, "actions638" : [ [ 6, 8, 3 ] ] }
{ "_id" : 6, "actions638" : [ [ 6, 3, 11 ], [ 3, 11, 8 ], [ 11, 8, 3 ] ] }
{ "_id" : 7, "actions638" : [ [ 6, 3, 8 ] ] }

Answer 1

$$this refers to the current item inside the array that is being processed by the $map function.

An alternative is to use the as property so that instead of referring to $$this you refer to the name you provided in the as. For example (from the docs)

db.grades.aggregate(
   [
      { $project:
         { adjustedGrades:
            {
              $map:
                 {
                   input: "$quizzes",
                   as: "grade",
                   in: { $add: [ "$$grade", 2 ] }
                 }
            }
         }
      }
   ]
)