As mentioned, sidebarMenu() must take in an id. For example if you pass in id = "tabs", then, anywhere in the server function, you can know which menuItem() (or menuSubItem()) is selected by accessing input$tabs. If you don’t pass in an id, you won’t be able access to this information. In addition, you need to specify a tabName for all “leaf” items (i.e. all menuItem()s or menuSubItem()s that don’t have any children), even if you wouldn’t need it otherwise. If you don’t specify a tabName, then that item will not be clickable, though the the other items will continue to function as normal (i.e. not specifying a tabName for an item is functionally the same as not having that item at all - except for the different visual look - since you won’t be able to interact with it at all).

This works exactly the same for dynamically generated sidebarMenu()s. Here’s an identical-looking app, whose only difference is that the sidebar menu is generated dynamically:

library(shiny)
library(shinydashboard)

ui <- dashboardPage(
  dashboardHeader(), 
  dashboardSidebar(
    sidebarMenuOutput("menu"),
    textOutput("res")
  ),
  dashboardBody(
    tabItems(
      tabItem("dashboard", "Dashboard tab content"),
      tabItem("widgets", "Widgets tab content"),
      tabItem("subitem1", "Sub-item 1 tab content"),
      tabItem("subitem2", "Sub-item 2 tab content") 
    )
  )
)

server <- function(input, output, session) {
  output$res <- renderText({
    paste("You've selected:", input$tabs)
  })
  output$menu <- renderMenu({
    sidebarMenu(
      # Setting id makes input$tabs give the tabName of currently-selected tab
      id = "tabs",

      menuItem("Dashboard", tabName = "dashboard", icon = icon("dashboard")),
      menuItem("Widgets", icon = icon("th"), tabName = "widgets"),
      menuItem("Charts", icon = icon("bar-chart-o"),
        menuSubItem("Sub-item 1", tabName = "subitem1"),
        menuSubItem("Sub-item 2", tabName = "subitem2")
      )
    )
  })
}

shinyApp(ui, server)

As you might have noticed by now, childless menuItem()s (and menuSubItem()s) and “childfull”" menuItem()s behave differently and there’s different arguments that apply to either:

• Childless menuItem()s/menuSubItem()s must be given a tabName argument; in addition, one of these can have selected = TRUE, which lets Shiny know that you want input$tabs (if you passed in id = "tabs" to sidebarMenu()) to start out with that item’s tabName (if you specified a tabItem() for that tabName, that will what you first see in the dashboard body when you launch your app).

• “Childfull” menuItem()s cannot have a tabName or a selected argument (or rather, they can, but this will be completely ignored by Shiny). Instead, at most, one of them can take a startExpanded = TRUE, which tells Shiny to start out with that menuItem() expanded, i.e. revealing all its children. (The reason that, at most, only one childfull menuItem() can start expanded is because in AdminLTE, only one thing can be expanded in the sidebar at any one time.) You can also pass in an expandedName, which is how Shiny lets you know which (if any) menuItem() is currently expanded (if expandedName is not provided, Shiny defaults to the vaue of the text argument, without any white space).

This is probably your first encounter with a Shiny input that has a fixed name. This is unusual, but it works just as well in this case (since there’s always only one sidebar, at most). It also has an advantage: since you get this input “for free” (i.e. you don’t have to pass an id or configure your app in any special way, other than making sure that your version of shinydashboard is equal or greater to 0.6), you also get bookmarking of this input “for free” (i.e. the bookmarked URL will capture which, if any, menuItem() was expanded and, on restore, it will expand the same one).