how to get length of arrayin mongo aggregation code example

Example 1: mongo count elements in array

{ $size: <expression> }

Example 2: mongodb count array size

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