I would like to create a download button within my shiny app to download a reactive plot created with chartJSRadar. I am not able to solve this problem! As I have been through the documented problem on the internet, I was not able to solve it, receiving all the time an empty png. As suggested (Save plots made in a shiny app), https://groups.google.com/forum/#!msg/shiny-discuss/u7gwXc8_vyY/IZK_o7b7I8gJ
I built a function ... So my code is an example code:

ui.R:

library(radarchart)

shinyUI(pageWithSidebar(
  headerPanel('Radarchart Shiny Example'),
  sidebarPanel(
    checkboxGroupInput('selectedPeople', 'Who to include', 
                       names(radarchart::skills)[-1], selected="Rich")
  ),
  mainPanel(
    chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7,
  radioButtons(inputId = "var3", label = "Select the file type", choices = list("png", "pdf")),
  downloadButton('downloadPlot', 'Download Plot')
  )
))

server.R

library(radarchart)

shinyServer(function(input, output) {
  output$plot1 <- renderChartJSRadar({

    chartJSRadar(skills[, c("Label", input$selectedPeople)], 
                 maxScale = 10, showToolTipLabel=TRUE)
  })

  plot2 <- function(){
    chartJSRadar(skills[, c("Label", input$selectedPeople)], 
                 maxScale = 10, showToolTipLabel=TRUE)
  }

  output$downloadPlot <- downloadHandler(
    filename = "Shinyplot.png",
    content = function(file) {
      png(file)
      plot2()
      print(plot2())
      dev.off()
  })    
})

Here is a JavaScript way which should be faster than webshot, I think.

library(shiny)
library(radarchart)
library(htmlwidgets) # to use the 'onRender' function

js <- c(
  "function(el, x){",
  "  $('#downloadPlot').on('click', function(){",
  "    // Clone the chart to add a background color.",
  "    var cloneCanvas = document.createElement('canvas');",
  "    cloneCanvas.width = el.width;",
  "    cloneCanvas.height = el.height;",
  "    var ctx = cloneCanvas.getContext('2d');",
  "    ctx.fillStyle = '#FFFFFF';",
  "    ctx.fillRect(0, 0, el.width, el.height);",
  "    ctx.drawImage(el, 0, 0);",
  "    // Download.",
  "    const a = document.createElement('a');",
  "    document.body.append(a);",
  "    a.download = 'radarchart.png';",
  "    a.href = cloneCanvas.toDataURL('image/png');",
  "    a.click();",
  "    a.remove();",
  "  });",
  "}"
)

ui <- pageWithSidebar(
  headerPanel('Radarchart Shiny Example'),
  sidebarPanel(
    checkboxGroupInput('selectedPeople', 'Who to include', 
                       names(radarchart::skills)[-1], selected="Rich"),
    actionButton('downloadPlot', 'Download Plot')
  ),
  mainPanel(
    chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7
  )
)

server <- function(input, output) {
  output$plot1 <- renderChartJSRadar({
    chartJSRadar(skills[, c("Label", input$selectedPeople)], 
                 maxScale = 10, showToolTipLabel=TRUE) %>% 
      onRender(js)
  })
}

shinyApp(ui, server)

This exports to png only. Use webshot to export to pdf.

EDIT

library(shiny)
library(radarchart)

js <- paste0(c(
  "$(document).ready(function(){",
  "  $('#downloadPlot').on('click', function(){",
  "    var el = document.getElementById('plot1');",
  "    // Clone the chart to add a background color.",
  "    var cloneCanvas = document.createElement('canvas');",
  "    cloneCanvas.width = el.width;",
  "    cloneCanvas.height = el.height;",
  "    var ctx = cloneCanvas.getContext('2d');",
  "    ctx.fillStyle = '#FFFFFF';",
  "    ctx.fillRect(0, 0, el.width, el.height);",
  "    ctx.drawImage(el, 0, 0);",
  "    // Download.",
  "    const a = document.createElement('a');",
  "    document.body.append(a);",
  "    a.download = 'radarchart.png';",
  "    a.href = cloneCanvas.toDataURL('image/png');",
  "    a.click();",
  "    a.remove();",
  "    cloneCanvas.remove();",
  "  });",
  "});"
), collapse = "\n")

ui <- pageWithSidebar(
  headerPanel('Radarchart Shiny Example'),
  sidebarPanel(
    checkboxGroupInput('selectedPeople', 'Who to include', 
                       names(radarchart::skills)[-1], selected="Rich"),
    actionButton('downloadPlot', 'Download Plot')
  ),
  mainPanel(
    tags$head(tags$script(HTML(js))),
    chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7
  )
)

server <- function(input, output) {
  output$plot1 <- renderChartJSRadar({
    chartJSRadar(skills[, c("Label", input$selectedPeople)], 
                 maxScale = 10, showToolTipLabel=TRUE) 
  })
}

shinyApp(ui, server)