Update value in a hidden field from R or from JavaScript using the Shiny package.

shiny-javascript-communication.R

library(shiny)
library(shinyjs)

ui <- fluidPage(
    useShinyjs(),
    hidden(
        textInput("my_input", label = "my_input", value = "Initial value")
    ),
    textOutput("my_output"),
    actionButton("my_js_button","Update from JavaScript"),
    actionButton("my_r_button", "Update from R"),
    tags$script("
    $(document).ready(function() {
      $('#my_js_button').click(function() {
         update_my_input_from_js('Updated data from JavaScript');
      });
    });
    function update_my_input_from_js(data) {
        alert('update_my_input_from_js: ' + data);
        Shiny.setInputValue('my_input', data);
    }
    Shiny.addCustomMessageHandler(type = 'send-data-from-r', function(message) {
        $('#my_output').html(message);
        alert('Shiny.addCustomMessageHandler:' + message);
    })
    ")
)

update_my_input_from_r <- function(session, data) {
    print(paste0("update_my_input_from_r: ", data))
    session$sendCustomMessage(
        type = "send-data-from-r",
        message = data
    )
}
server <- function(input, output, session) {
    output$my_output <- renderText({
        input$my_input
    })
    observeEvent(input$my_r_button, {
        update_my_input_from_r(session, data = "Updated data from R")
    })
}

shinyApp(ui, server)