R/showLog.R

In shinyjs: Easily Improve the User Experience of Your Shiny Apps in Seconds

#' Print any JavaScript console.log messages in the R console
#'
#' When developing and debugging a Shiny that uses custom JavaScript code,
#' it can be helpful to use \code{console.log()} messages in JavaScript. This
#' function allows you to see these messages printed in the R console directly
#' rather than having to open the JavaScript console in the browser to view the
#' messages.\cr\cr
#' This function must be called in a Shiny app's server.
#' @note Log messages that cannot be serialized in JavaScript (such as many
#' JavaScript Event objects that are cyclic) will not be printed in R.
#' @examples
#' if (interactive()) {
#'   library(shiny)
#'
#'   shinyApp(
#'     ui = fluidPage(
#'       useShinyjs(),
#'       textInput("text", "Type something")
#'     ),
#'     server = function(input, output) {
#'       showLog()
#'       logjs("App started")
#'       observe({
#'         logjs(paste("Length of text:", nchar(input$text)))
#'       })
#'     }
#'   )
#' }
#' @seealso [logjs()]
#' @export
showLog <- function() {
  session <- getSession()

  if (!is.null(attr(session, "shinyjs_showLog"))) {
    return()
  }
  attr(session, "shinyjs_showLog") <- TRUE

  # Capture the console.log function and overwrite it to send the message to R
  shiny::insertUI("head", "beforeEnd", {
    shiny::singleton(shiny::tags$head(shiny::tags$script(
      '(function(){
        var oldLog = console.log;
        var queue = new ShinySenderQueue();
        console.log = function (message) {
          try {
            queue.send("shinyjs-showLog", message);
          } catch(err) {}
          oldLog.apply(console, arguments);
        };
      })();'
    )))
  }, immediate = TRUE)

  shiny::observeEvent(session$input[['shinyjs-showLog']], {
    message("[JAVASCRIPT LOG] ",
            jsonlite::toJSON(session$input[['shinyjs-showLog']],
                             auto_unbox = TRUE)
    )
  })
}