How to use mongoose auto increment inside a array in mongoDB

now in mongoDB the default _id for array objects are ObjectId('62e10eb9150cd328edd6f6y3')

This is not true, this is true for mongoose as it generates an ObjectId for each object by default - but this is not a MongoDB feature.

mongoose does not offer any alternatives as a feature, in your case using index of array as _id, but you can work around it by executing a slightly less efficient update using the aggregation pipeline syntax, like so:

const newArrayItem = { ... new array items };

db.collection.updateOne(
{},
[
  {
    "$set": {
      "items": {
        $concatArrays: [
          {
            $ifNull: [
              "$items",
              []
            ]
          },
          [
            {
              $mergeObjects: [
                newArrayItem,
                {
                  _id: {
                    $size: {
                      $ifNull: [
                        "$items",
                        []
                      ]
                    }
                  }
                }
              ]
            }
          ]
        ]
      }
    }
  }
])

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