How to access browser session/cookies from within Shiny App

Accessing browser sessions or cookies in Shiny applications typically requires leveraging JavaScript and Shiny's server communication capabilities. Since Shiny is built on R, and R itself lacks direct functionality to operate on browser cookies, we need to leverage JavaScript for this purpose.

Below, I will detail how to implement this functionality in a Shiny App:

1. Embedding JavaScript Code in the UI

First, we need to embed JavaScript code in the Shiny UI section (typically in ui.R or the corresponding UI definition) for reading cookies. We can directly embed JavaScript code into the page using tags$script().

R
library(shiny)

ui <- fluidPage(
    tags$head(
        tags$script(HTML(" 
            function getCookie(name) {
                var value = '; ' + document.cookie;
                var parts = value.split('; ' + name + '=');
                if (parts.length == 2) return parts.pop().split(';').shift();
            }
            
            Shiny.addCustomMessageHandler('getCookie', function(name) {
                var cookieValue = getCookie(name);
                Shiny.setInputValue('cookieValue', cookieValue);
            });
        "))
    ),
    verbatimTextOutput("cookieOutput")
)

2. Server-Side Code

On the server side (server.R or the corresponding server logic), we can send custom messages to the frontend to trigger JavaScript functions and retrieve cookie values.

R
server <- function(input, output, session) {

    observe({
        session$sendCustomMessage(type = 'getCookie', message = 'userSession')
    })

    output$cookieOutput <- renderText({
        paste("Cookie value:", input$cookieValue)
    })
}

3. Running the Shiny Application

Finally, use shinyApp(ui = ui, server = server) to run the entire application.

Example:

Suppose we need to retrieve the cookie named userSession. The above code demonstrates how to obtain this cookie from the browser and display it within the Shiny App. With the JavaScript function getCookie, we can read any existing cookie. Of course, the specific JavaScript code may need to be adjusted based on the actual cookie name and structure.

By this approach, you can flexibly access and utilize browser-side data within Shiny applications, enabling richer user interactions and personalized features.