add

Adds key-value pairs from other to this. Returns the original receiver.

fun Json.add(other: Json): Json(source)

import kotlin.js.Json

val base: Json = js("{x: 10}")
val other: Json = js("{y: 20, z: 30}")

base.add(other)

println(JSON.stringify(base))  // {"x":10,"y":20,"z":30}

Source