Shiny conditionalPanel conditions based on NULL values in R? - r

Question

I am writing a Shiny application that will query a database a few times. The queries may take some time, so I am using actionButton to allow the user to control when they are started.
The general flow of the application is:
User loads page, pushes button to load in available choices from the database.
User selects a row from the available choices, and then kicks off a larger query that will take longer.
When the query is done, the user will get nice visualizations and other such things.
I know that you can allow the user to select rows from a DataTable using the selection option, and I am currently using the development build so that single-selection can be enabled. My problem is figuring out how to hide certain conditional panels until the user has made a choice. As far as I can tell, the value of the choice is stored in input$[data frame name]_rows_selected. But, until the values are selected, this value is either NULL, or does not exist.
I cannot figure out how to pass to the condition argument of conditionalPanel() a condition which reflects the internal R logic. I've written a minimal working example below which shows the behavior to which I'm referring.
library(shiny)
library(DT)
# Create sample data
df_sample_data <- data.frame(name = c("John Smith","Jane Cochran","Belle Ralston","Quincy Darcelio"),
color = c("Red","Blue","Red","Green"),
age = c(25,52,31,29))
ui <-
fluidPage(
titlePanel("The Title!"),
sidebarPanel(
h1("Load Data"),
actionButton("load_data","Load Data"),
br(),
conditionalPanel(
h1("Do Thing"),
actionButton("do_thing","Do Thing"),
condition = "input.df_data_rows_selected !== undefined")
),
mainPanel(
dataTableOutput("df_data"),
conditionalPanel(
tableOutput("row_selected"),
condition = "input.df_data_rows_selected !== undefined")
)
)
server <-
function(input, output) {
# This function loads the data (in reality it is a server operation)
uFunc_ReactiveDataLoad <- eventReactive(eventExpr = input$load_data,valueExpr = {
df_data <- df_sample_data
return(list(display_table = datatable(data = df_data[c("name","age")],
options = list(dom = "tip"),
filter = "top",
selection = "single",
colnames = c("Person Name" = "name",
"Person Age" = "age")),
data_table = df_data))
})
output$df_data <- renderDataTable(expr = uFunc_ReactiveDataLoad()$display_table)
output$row_selected <- renderTable(expr = uFunc_ReactiveDataLoad()$data_table[input$df_data_rows_selected,])
}
shinyApp(ui = ui, server = server)
In the current setup, which uses input.df_data_rows_selected !== undefined, the panels are hidden until the data is loaded using the first actionButton. But, I need them to remain hidden unless the user has selected a row. I have tried other things such as:
input.df_data_rows_selected !== null
input.df_data_rows_selected !== 'null'
input.df_data_rows_selected !== ''
input.df_data_rows_selected !== 'NULL'
...and so on, but I have had no luck. How does the NULL value in R get represented in the JavaScript used for the condition argument to conditionalPanel()?

Answer 1

condition = "(typeof input.df_data_rows_selected !== 'undefined' && input.df_data_rows_selected.length > 0)" for both entries seems to work.

Answer 2

condition = "(typeof input.df_data_rows_selected === null")
If anyone is stumbling upon this issue a few years later...