play-json-default-value.scala

case class Person1(name: String, age: Int, other: Option[String])
case class Person2(name: String, age: Int, other: String)

import play.api.libs.functional.syntax._
import play.api.libs.json._

implicit val person1Format = (
  (JsPath \ "name").format[String] and
    (JsPath \ "age").format[Int] and
    (JsPath \ "other").formatNullable[String].inmap[Option[String]](x => Some(x.getOrElse("default")), y => y)
  ) (Person1.apply, unlift(Person1.unapply))

val str1 =
  """
    |{"name":"x", "age":18}
  """.stripMargin

val str2 =
  """
    |{"name":"x", "age":18, "other":"test"}
  """.stripMargin

println(Json.parse(str1).as[Person1])
println(Json.parse(str2).as[Person1])

implicit val person2Format = (
  (JsPath \ "name").format[String] and
    (JsPath \ "age").format[Int] and
    (JsPath \ "other").formatNullable[String].inmap[String](_.getOrElse("default"), Some(_))
  ) (Person2.apply, unlift(Person2.unapply))

println(Json.parse(str1).as[Person2])
println(Json.parse(str2).as[Person2])


// Output:
// Person1(x,18,Some(default))
// Person1(x,18,Some(test))
// Person2(x,18,default)
// Person2(x,18,test)