How to use $lookup $let variables in mongodb?
You can only convert req.user._id using mongoose.Types.ObjectId as it becomes a constant value for Aggregation Framework. To convert a variable $$content you have to use $toObjectId operator.
{"$lookup": { "from": "likes", "let": {'content': "$_id" }, "pipeline": [ {'$match': { $expr: { $and: [ { $eq: [ "$user", mongoose.Types.ObjectId(req.user._id) ] }, { $eq: [ "$content", { $toObjectId: "$$content" } ] }, ] } } } ], "as": "liked"}},