library(data.tree)

#create an example tree
contacts <- Node$new("contacts")
contacts$type <- "root"
jack <- contacts$AddChild("c1")
jack$fullName <- "Jack Miller"
jack$isGoodCustomer <- FALSE
jack$type <- "customer"
jill <- contacts$AddChild("c2")
jill$fullName <- "Jill Hampsted"
jill$isGoodCustomer <- TRUE
jill$type <- "customer"
o1 <- jill$AddChild("o1")
o1$type <- "order"
o1$item <- "Shoes"
o1$amount <- 29.95

#This function will convert the Node objects to environments
EnvConverter <- function(node) {
  #We take env and not list, because list has value semantics (just try it with list!)
  me <- new.env()
  if (node$type == "customer") {
    #here you decide which fields you'll want in the JSON
    #you could also format, transform, etc.
    me$fullName <- node$fullName
    me$isGoodCustomer <- node$isGoodCustomer
  } else if (node$type == "order") {
    me$item <- node$item
    me$amount <- node$amount
  } else {
    me$name <- node$name
  }

  if (!node$isRoot) {
    node$parent$json[[node$name]] <- me
  }
  node$json <- me
  #dummy return (not needed)
  return (node$name)
}

#iterate through the tree and call EnvConverter
contacts$Get(EnvConverter)

#needed to convert the above created environment to a list
ConvertNestedEnvironmentToList <- function(env) {
  out <- as.list(env)
  lapply(out, function(x) if (is.environment(x)) ConvertNestedEnvironmentToList(x) else x)
}

mylist <- ConvertNestedEnvironmentToList(contacts$json)

library(rjson)

#convert the list to a JSON, using the package of your choice
toJSON(mylist)