get shiny input values on the frontend

cut unnecessary trips to the server

r
shiny
javascript
ui/ux
Author

Kennedy Mwavu

Published

July 25, 2025

Modified

August 15, 2025

tl;dr: Shiny.shinyapp.$inputValues["inputValueId"]

intro

there are some actions which can be performed purely on the frontend without an extra trip to your server.

let’s focus on this example:

you have two radio buttons: csv, and pdf. beside them, you have a download button. when clicked:

  • if csv option is the chosen one, the label on the download button becomes “Downloading…”
  • if it’s the pdf option, the label on the download button becomes “Rendering PDF…”

this can [and should] be performed on the frontend. a fairly small amount of jquery gets the job done. no unnecessary trip to the server.

anyone who reads my posts is now like “oh, no! not him yapping about download buttons again :(”

the meme 'even harder' but with the caption 'stop talking about download buttons' and 'you know what? i'm gonna talk about them even harder

reprex

let’s setup a minimal reprex first.

app.R
library(shiny)
library(bslib)

ui <- page(
  version = bs_theme(version = 5L),
  title = "shiny input values",
  lang = "en",
  tags$div(
    class = "container",
    tags$h3("get shiny input values on the frontend"),
    tableOutput(outputId = "iris"),
    radioButtons(
      inputId = "filetype",
      label = NULL,
      choices = c(CSV = "csv", PDF = "pdf"),
      selected = "csv",
      inline = TRUE
    ),
    downloadButton(
      outputId = "download_file",
      label = "Download",
      class = "btn-sm btn-outline-dark"
    )
  )
)

server <- \(input, output, session) {
  output$iris <- renderTable(head(iris))

  output$download_file <- downloadHandler(
    filename = \() {
      paste0("iris.", input$filetype)
    },
    content = \(file_path) {
      Sys.sleep(3) # <-- simulate long download

      switch(
        EXPR = input$filetype,
        csv = write.csv(x = head(iris), file = file_path),
        pdf = file.copy(from = "iris.pdf", to = file_path)
      )
    }
  )
}

shinyApp(ui, server)

since we aren’t going to do real rendering for the pdf, copy this pdf to the same dir as app.R.

the reprex is complete!

downloading spinners

obviously, we have to do this. it’s courteous to let your users know that something is happening.

after the download button is clicked, we want it’s label to be dependent on the value of input$filetype. the easiest way to do this is:

  1. when app starts, send a custom message to the frontend with two items:
    • input ID of the radio buttons, and
    • input ID of the download button
  2. on the frontend:
    • add a custom handler for that message, and
    • attach an on-click event to the download button

send custom message

we’ll use the session object in \(input, output, session) to send a custom message to the frontend once our app starts.

app.R
session$sendCustomMessage(
  type = "update-download-button",
  list(
    radioButtonId = session$ns("filetype"),
    downloadButtonId = session$ns("download_file")
  )
)

note:

  • used camelCase for the item labels since it’s what i mostly use when writing js.
  • namespaced the element IDs via session$ns(). ensures this will work even in shiny modules.

now app.R looks like this:

Code 
app.R
library(shiny)
library(bslib)

ui <- page(
  version = bs_theme(version = 5L),
  title = "shiny input values",
  lang = "en",
  tags$div(
    class = "container",
    tags$h3("get shiny input values on the frontend"),
    tableOutput(outputId = "iris"),
    radioButtons(
      inputId = "filetype",
      label = NULL,
      choices = c(CSV = "csv", PDF = "pdf"),
      selected = "csv",
      inline = TRUE
    ),
    downloadButton(
      outputId = "download_file",
      label = "Download",
      class = "btn-sm btn-outline-dark"
    )
  )
)

server <- \(input, output, session) {
  session$sendCustomMessage(
    type = "update-download-button",
    list(
      radioButtonId = session$ns("filetype"),
      downloadButtonId = session$ns("download_file")
    )
  )

  output$iris <- renderTable(head(iris))

  output$download_file <- downloadHandler(
    filename = \() {
      paste0("iris.", input$filetype)
    },
    content = \(file_path) {
      Sys.sleep(3) # <-- simulate long download

      switch(
        EXPR = input$filetype,
        csv = write.csv(x = head(iris), file = file_path),
        pdf = file.copy(from = "iris.pdf", to = file_path)
      )
    }
  )
}

shinyApp(ui, server)

custom message handler

create a new file in ./public/main.js:

./public/main.js
Shiny.addCustomMessageHandler("update-download-button", function (msg) {
  const downloadButton = $("#" + msg.downloadButtonId);

  downloadButton.on("click", function () {
    const fileType = Shiny.shinyapp.$inputValues[msg.radioButtonId];
    const text = fileType === "csv" ? "Downloading..." : "Rendering PDF...";
    const spinner =
      '<span class="spinner-border spinner-border-sm" aria-hidden="true"></span>' +
      `<span role="status"> ${text} </span>`;

    downloadButton.html(spinner);
    downloadButton.attr("aria-disabled", true);
    downloadButton.addClass("disabled");
  });
});

Shiny.shinyapp.$inputValues["inputValueId"] is how you access shiny input values on the frontend. it’s the equivalent of input$inputValueId on the server.

to include this js file in your app:

  • ensure that the ./public/ dir is added as a resource path:

    app.R
    addResourcePath(prefix = "static", directoryPath = "public")

  • use tags$script() at the end of your ui.

    app.R
    tags$script(src = "/static/main.js")

after this, app.R will now be:

Code 
app.R
library(shiny)
library(bslib)

addResourcePath(prefix = "static", directoryPath = "public")

ui <- page(
  version = bs_theme(version = 5L),
  title = "shiny input values",
  lang = "en",
  tags$div(
    class = "container",
    tags$h3("get shiny input values on the frontend"),
    tableOutput(outputId = "iris"),
    radioButtons(
      inputId = "filetype",
      label = NULL,
      choices = c(CSV = "csv", PDF = "pdf"),
      selected = "csv",
      inline = TRUE
    ),
    downloadButton(
      outputId = "download_file",
      label = "Download",
      class = "btn-sm btn-outline-dark"
    )
  ),
  tags$script(src = "/static/main.js")
)

server <- \(input, output, session) {
  session$sendCustomMessage(
    type = "update-download-button",
    list(
      radioButtonId = session$ns("filetype"),
      downloadButtonId = session$ns("download_file")
    )
  )

  output$iris <- renderTable(head(iris))

  output$download_file <- downloadHandler(
    filename = \() {
      paste0("iris.", input$filetype)
    },
    content = \(file_path) {
      Sys.sleep(3) # <-- simulate long download

      switch(
        EXPR = input$filetype,
        csv = write.csv(x = head(iris), file = file_path),
        pdf = file.copy(from = "iris.pdf", to = file_path)
      )
    }
  )
}

shinyApp(ui, server)

now when the download button is clicked, its label is updated depending on the value of input$filetype.

final step: once the download is done, remove the spinner and revert back to the original label of the download button.

to do that:

  • send a custom message once downloadHandler() exits.
  • add a handler for that message on the frontend.
app.R
on.exit({
  session$sendCustomMessage(
    type = "end-download",
    list(
      downloadButtonId = session$ns("download_file")
    )
  )
})

this updates app.R to this:

Code 
app.R
library(shiny)
library(bslib)

addResourcePath(prefix = "static", directoryPath = "public")

ui <- page(
  version = bs_theme(version = 5L),
  title = "shiny input values",
  lang = "en",
  tags$div(
    class = "container",
    tags$h3("get shiny input values on the frontend"),
    tableOutput(outputId = "iris"),
    radioButtons(
      inputId = "filetype",
      label = NULL,
      choices = c(CSV = "csv", PDF = "pdf"),
      selected = "csv",
      inline = TRUE
    ),
    downloadButton(
      outputId = "download_file",
      label = "Download",
      class = "btn-sm btn-outline-dark"
    )
  ),
  tags$script(src = "/static/main.js")
)

server <- \(input, output, session) {
  session$sendCustomMessage(
    type = "update-download-button",
    list(
      radioButtonId = session$ns("filetype"),
      downloadButtonId = session$ns("download_file")
    )
  )

  output$iris <- renderTable(head(iris))

  output$download_file <- downloadHandler(
    filename = \() {
      paste0("iris.", input$filetype)
    },
    content = \(file_path) {
      on.exit({
        session$sendCustomMessage(
          type = "end-download",
          list(
            downloadButtonId = session$ns("download_file")
          )
        )
      })

      Sys.sleep(3) # <-- simulate long download

      switch(
        EXPR = input$filetype,
        csv = write.csv(x = head(iris), file = file_path),
        pdf = file.copy(from = "iris.pdf", to = file_path)
      )
    }
  )
}

shinyApp(ui, server)

in ./public/main.js we’ll add the handler:

./public/main.js
Shiny.addCustomMessageHandler("end-download", function (msg) {
  const downloadButton = $("#" + msg.downloadButtonId);
  const innerHtml =
    '<i class="fas fa-download" role="presentation" aria-label="download icon"></i> Download ';

  downloadButton.html(innerHtml);
  downloadButton.attr("aria-disabled", false);
  downloadButton.removeClass("disabled");
});

the final ./public/main.js is now:

Code 
./public/main.js
Shiny.addCustomMessageHandler("update-download-button", function (msg) {
  const downloadButton = $("#" + msg.downloadButtonId);

  downloadButton.on("click", function () {
    const fileType = Shiny.shinyapp.$inputValues[msg.radioButtonId];
    const text = fileType === "csv" ? "Downloading..." : "Rendering PDF...";
    const spinner =
      '<span class="spinner-border spinner-border-sm" aria-hidden="true"></span>' +
      `<span role="status"> ${text} </span>`;

    downloadButton.html(spinner);
    downloadButton.attr("aria-disabled", true);
    downloadButton.addClass("disabled");
  });
});

Shiny.addCustomMessageHandler("end-download", function (msg) {
  const downloadButton = $("#" + msg.downloadButtonId);
  const innerHtml =
    '<i class="fas fa-download" role="presentation" aria-label="download icon"></i> Download ';

  downloadButton.html(innerHtml);
  downloadButton.attr("aria-disabled", false);
  downloadButton.removeClass("disabled");
});

conclusion

atwood's law: any application that can be written in javascript will eventually be written in javascript