updateNullable

It would be great if JsPath supported the ability to update a path if it exists. Currently, if you try to define an update on a path that doesn't exist, you'll get an error. Here's a helper that does this:

private implicit class PathUpdateNullable(path: JsPath) {
    def updateNullable[A <: JsValue](reads: Reads[A]) =
      Reads[JsObject] {
        case o: JsObject =>
          path.asSingleJson(o) match {
            case JsDefined(jsv) =>
              val updated = reads.reads(jsv).repath(path)
              updated.map { updatedJsv =>
                o.deepMerge(JsPath.createObj(path -> updatedJsv))
              }
            case _ => JsSuccess(o)
          }
        case _ =>
          JsError(JsPath, JsonValidationError("error.expected.jsobject"))
      }
  }

Start with the JsPath update behavior and compare it with the updateNullable helper shown in the issue. Trace how a missing path is handled and identify the existing JsPath entry points involved. Done means updates succeed when the path exists and leave the object unchanged when it does not.