[{"product":"p1","salesdate":ISODate("2021-02-01T00:00:00Z"),"amount":100},{"product":"p2","salesdate":ISODate("2021-02-04T00:00:00Z"),"amount":200},]

db.collection.aggregate([{$group:{_id:null,data:{$push:"$$ROOT"}}},{$addFields:{data:{$reduce:{input:{$range:[0,5]},initialValue:[],in:{$let:{vars:{ts:{$add:[ISODate("2021-02-01T00:00:00Z"),{$multiply:["$$this",1000,60,60,24]}]}},in:{$concatArrays:["$$value",[{$ifNull:[{$first:{$filter:{input:"$data",cond:{$eq:["$$this.salesdate","$$ts"]}}}},{salesdate:"$$ts",amount:0}]}]]}}}}}}},{$unwind:"$data"},{$replaceRoot:{newRoot:"$data"}}])