library(shiny)
library(shinyjs)

if (interactive()) {
     ui <- shinyUI(
    fluidPage(
      useShinyjs(),
      tags$head(tags$title("Title"),
                tags$link(rel = "stylesheet", href = "codemirror.css"),
                tags$link(rel = "stylesheet", href = "cobalt.css"),
                tags$script(src = "codemirror.js"),
                tags$script(src = "r.js")
      ),
      actionButton("btn1","Click to see code"),
      uiOutput(outputId = "textStringToDisplay")))
     server <- function(input, output){
    output$textStringToDisplay <- renderUI(
      tags$textarea(id="textBox", name = "Feedback", paste0(sample(letters,15),collapse = "")))

    ButtonPressCounter <- 0

    observeEvent(input$btn1,
                 {
                   ButtonPressCounter <<- ButtonPressCounter + 1 # Need it to happen only once
                   if(ButtonPressCounter <= 1){
                     shinyjs::runjs(
                       'var editorR = CodeMirror.fromTextArea(textBox, {
                       mode: "r",
                       lineNumbers: true,
                       smartindent: true});
                       editorR.setOption("theme", "cobalt");
                       editorR.setSize("100%","100%");')
                   }
                 })
       }
     shinyApp(ui = ui, server = server) }