Why will upickle convert `Option[T]` to array `[...]`?

I found if there is an option value in case class, it will be converted to [] in js

case class User(name:String, age: Option[Int])

val user1 = User("Free", Some(100))
val user2 = User("Wind", None)

Will be converted to:

{ "name": "Free", "age": [100] }
{ "name": "Wind", "age": [] }

Why use array here rather than:

{ "name": "Free", "age": 100 }
{ "name": "Wind" }

The later is the one I usually do with other scala json libraries

[l15k4]

This writer is applied for Some(v) :

  implicit val optHavingWriter = upickle.default.Writer[Option[HavingSpec]]{
    case Some(v) => writeJs[HavingSpec](v)
    case None => Js.Null
  }

This one too :

  implicit val optBooleanWriter = upickle.default.Writer[Option[Boolean]]{
    case Some(v) => writeJs[Boolean](v)
    case None => Js.Null
  }

But these are not :

  implicit val optStringWriter = upickle.default.Writer[Option[String]]{
    case Some(v) => writeJs[String](v)
    case None => Js.Null
  }
  implicit val optIntWriter = upickle.default.Writer[Option[Int]]{
    case Some(v) => writeJs[Int](v)
    case None => Js.Null
  }

Unfortunately generic types doesn’t work at all, so I have to list a custom writer for each type :

  implicit def optWriter[T : Writer] = upickle.default.Writer[Option[T]]{
    case Some(v) => writeJs[T](v)
    case None => Js.Null
  }

It would be cool if it worked even for String and Int … any idea ?

[mycaule]

One common use case would be a reader for

case class MyCC(a: String, b: String, c: Option[String])

so that undefined is parsed too :

// expected
{"a": "x", "b": "y"}           => MyCC("x", "y", None)
// upickle.core.AbortException: missing keys in dictionary: c

// expected and verified
{"a": "x", "b": "y", "c": "z"} => MyCC("x", "y", Some("z"))

Last solution from @asdcdow does this

{"a": "x", "b": "y", "c": null} => MyCC("x", "y", None)

Modifying MyCC with Defaults

case class MyCC(a: String, b: String, c: Option[String] = None)

I get what I expect

// expected and verified
{"a": "x", "b": "y"}           => MyCC("x", "y", None)