library(shiny)
library(shinyjs)

ui<-shinyUI(
  fluidPage(
    useShinyjs(),
    tags$form(
      class = "form-horizontal", action="/action_page.php",

      div(
        class = 'form-group',
        tags$label(class = 'control-label col-sm-2', `for` = 'month_abb', 'Month (abbr.)'),
        div(
          class = 'col-sm-4',
          selectInput('month_abb', '', month.abb)
        )
      ),

      div(
        class = 'form-group',
        tags$label(class = 'control-label col-sm-2', `for` = 'month_full', 'Month (full)'),
        div(
          class = 'col-sm-4',
          selectInput('month_full', '', month.name)
        )
      ),

      checkboxInput('abbr', 'Abbreviated')

    )
  )
)

server<-function(input, output) {

  observe({

    toggle(selector = "$('#month_full').parent('.form-group')", condition = !input$abbr)

    toggle(selector = "$('#month_abbr').parent('.form-group')", condition = input$abbr)

  })

}


shinyApp(ui,server)