Accepted answer

JsObject has a + method that allows you to add fields to an object, but unfortunately your jsonObject is statically typed as a JsValue, not a JsObject. You can get around this in a couple of ways. The first is to use as:

 scala>[JsObject] + ("c" -> Json.toJson(3))
 res0: play.api.libs.json.JsObject = {"a":1,"b":2,"c":3}

With as you're essentially downcasting—you're telling the compiler, "you only know that this is a JsValue, but believe me, it's also a JsObject". This is safe in this case, but it's not a good idea. A more principled approach is to use the OWrites directly:

scala> val jsonObject = classAWrites.writes(classAObject)
jsonObject: play.api.libs.json.JsObject = {"a":1,"b":2}

scala> jsonObject + ("c" -> Json.toJson(3))
res1: play.api.libs.json.JsObject = {"a":1,"b":2,"c":3}

Maybe someday the Json object will have a toJsonObject method that will require a OWrites instance and this overly explicit approach won't be necessary.


simpler way is to use argoanut (

var jField : Json.JsonField = "myfield" //Json.JsonField is of type String
obj1.asJson.->:(jField, obj2.asJson)  // adds a field to obj1.asJson

here obj1.asJson creates a JSON object and obj2 is the object to be added to the json created by obj1.asJson


I found a solution myself. In fact the JsValue, which is the return type of Json.toJson has no such method, but the JsObject ( does, so the solution is:

val jsonObject = Json.toJson(classAObject).as[JsObject]
jsonObject + ("c", JsNumber(3)) 

I hope someone will find this useful :)

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