Converting JsValue to String

Your error comes from the fact that you don't impose enough condition on x's type : maybeString is an Option[JsValue], not Option[JsString]. In the case maybeString is not an Option[JsString], the conversion fails and raises and exception.

You could do this :

val str: String = maybeString match {
  case Some(x:JsString) => x.as[String]
  case _       => "0"
}

Or you could use asOpt[T] instead of as[T], which returns Some(_.as[String]) if the conversion was successful, None otherwise.

You want to compose multiple Options, that's what flatMap is for:

maybeString flatMap { json =>
  json.asOpt[String] map { str =>
    // do something with it
    str
  }
} getOrElse "0"

Or as a for comprehension:

(for {
  json <- maybeString
  str <- json.asOpt[String]
} yield str).getOrElse("0")

I'd also advise to work with the value inside the map and pass the Option around, so a None will be handled by your controller and mapped to a BadRequest for example.