Why do we use session$ns in Shiny modules and not NS(id)?

Consider any Shiny module in which we use session$ns in the server part, e.g. the one below.

We could equivalently use NS(id) instead of session$ns. So why it is recommended to use session$ns? Is there an advantage over NS(id)?

library(shiny)

myModuleUI <- function(id){
  ns <- NS(id)
  uiOutput(ns("checkbox"))
}

myModuleServer <- function(id){
  moduleServer(id, function(input, output, session){
    ns <- session$ns
    output[["checkbox"]] <- renderUI({
      checkboxInput(ns("checkbox"), label = "Check me", value = FALSE)
    })
    observe({
      print(input[["checkbox"]])
    })
  })
}

ui <- basicPage(
  myModuleUI("myModule")
)

server <- function(input, output, session){
  myModuleServer("myModule")
}

shinyApp(ui, server)

Solution

Why?

session$ns is mandatory if you want to use nested modules which use dynamic UI with uiOutput

Minimal example

library(shiny)
## NESTED MODULE
nestedModuleUI <- function(id) {
  ns <- NS(id)
  uiOutput(ns("ui_out"))
}

nestedModuleServer <- function(id) {
  moduleServer(
    id,
    function(input, output, session) {

      output$ui_out <- renderUI({
        list(
          actionButton(session$ns("well_formed"), "I work"),
          actionButton(NS(id, "bad_formed"), "I do nothin'")
        )
      })

      observeEvent(input$well_formed,  print("button pressed!"))
      observeEvent(input$bad_formed,  print("button pressed!"))
    }
  )
}

## CONTAINER MODULE
containerModuleUI <- function(id) {
  ns <- NS(id)
  nestedModuleUI(ns("nested"))
}

containerModuleServer <- function(id) {
  moduleServer(
    id,
    function(input, output, session) {
      nestedModuleServer("nested")
    }
  )
}

## Application
ui <-  containerModuleUI("container")

server <- function(input, output, session) containerModuleServer("container")

shinyApp(ui, server)