how to find array length in mongodb code example

Example 1: mongodb filter empty array

db.collection.find({arrayElementName : {$exists:true, $size:0}})

Example 2: mongodb count array size

db.inventory.aggregate([
   {
      $project: {
         item: 1,
         numberOfColors: { $cond: { if: { $isArray: "$colors" }, then: { $size: "$colors" }, else: "NA"} }
      }
   }
] )