shiny Javascript API Sending data from server to client

Example

In many instances, you will want to send data from the R server to the JS client. Here is a very simple example:

library(shiny)
runApp(
  list(
    ui = fluidPage(
      tags$script(
        "Shiny.addCustomMessageHandler('message', function(params) { alert(params); });"  
      ),
      actionButton("btn","Press Me")
    ),
    server = function(input, output, session) {
      observeEvent(input$btn,{
        randomNumber <- runif(1,0,100)
        session$sendCustomMessage("message",list(paste0(randomNumber," is a random number!")))
      })
    }
  )
)

The workhorses here are the session$sendCustomMessage function in R and the Shiny.addCustomMessageHandler function in javascript.

The session$sendCustomMessage function lets you send parameters from R to a javascript function, and Shiny.addCustomMessageHandler let's you define the javascript function that accepts the parameters from R.

Note: Lists are converted to JSON when they are passed from R to javascript