Minimal example illustrating a delay in communicating between JS and Shiny.

js-r-delay-example.R

get_text <- function() {
  session <- shiny::getDefaultReactiveDomain()
  session$sendCustomMessage("get_text", list())
}

# append the text from a given ID with the result of "get_text" from JS
append_text <- function(id) {
  get_text()
  session <- shiny::getDefaultReactiveDomain()
  paste(session$input[[id]], session$input$testText)
}

library(shiny)

ui <- fluidPage(
  tags$script("
              Shiny.addCustomMessageHandler('get_text', function(params) {
              
              let text = $('#test-text').html();
              
              if (typeof(text) !== 'undefined') {message = text;} else {message = 'NA';}
              
              Shiny.onInputChange('testText', message);
              
              });
              "),
  p(id = "test-text", "appending this"), # define a p tag with id "test-text" that is read by the JS function
  textInput("userText", "Input Text"),
  actionButton("getText", "Get Text"),
  verbatimTextOutput("appended")
)

server <- function(input, output, session) {
  
  text <- reactiveVal()
  
  # when action button "getText" is pressed, call `append_text()`
  # and set the reactive value text equal to the results
  observeEvent(input$getText, {
    text(append_text(id = "userText"))
  })
  
  # render the reactive value text.
  # takes two clicks for the appending to work correctly.
  output$appended <- renderText({
    text()
  })
}

shinyApp(ui, server)