Easy way to show an error: use javascript alert from server.R

[Dmitri]

I often need to alert the user that nothing was run because of incorrect input. Instead of trying to show an error message inside a data.frame using renderDataTable(), for example, or keeping a separate output object for that, I came up with a way to show a javascript alert from server.R. Here is an example for those unfamiliar with addCustomMessageHandler(), who need similar functionality.

Add the following to ui.R:

tags$script(type="text/javascript", "

   $(document).ready(function() {

        Shiny.addCustomMessageHandler('showalert', function(message) {

          alert(message);

        });

   });

"),

Use inside server.R:

session$sendCustomMessage(type = "showalert", "error message here")