Shiny input$____ value when no values are selected in selectizeInput

Question

I have an shiny app that looks like this

ui.R

shinyUI(fluidPage(

            titlePanel("Test Application"),

            sidebarLayout(
                    sidebarPanel(

                            selectizeInput("N",
                                    label = ("N"),
                                    multiple = TRUE,
                                    choices = NULL,
                                    options = list(
                                            placeholder = 'Select All Desired, Type to Search',
                                            onInitialize = I('function() { this.setValue(""); }')
                                    )),

                            selectizeInput("M", 
                                    label = "M",
                                    multiple = TRUE,
                                    choices = NULL,
                                    options = list(
                                            placeholder = 'Select All Desired, Type to Search',
                                            onInitialize = I('function() { this.setValue(""); }')
                                    ))
                    ),

                    mainPanel(
                            tabsetPanel(
                                    tabPanel("Test Plot 1",
                                            plotOutput("testPlot1"))                                        
                    )
            ))))

server.R

library(data.table)
library(ggplot2)
options("stringsAsFactors" = FALSE, datatable.verbose=TRUE, datatable.auto.index=TRUE)

testDT <- data.table(
        L = (1:32), 
        M = rep(letters[23:26], each = 64), 
        N = rep(LETTERS[1:4], times = 20, each = 512),
        O = rnorm(2048, 1))

testDT$L                <- factor(testDT$L,         levels = seq(from = 1, to = 32, by = 1))

shinyServer(function(input, output, session) {

        updateSelectizeInput(session, "N", 
                server = TRUE, 
                choices = sort(unique(testDT$N)),
        )

        updateSelectizeInput(session, "M", 
                server = TRUE, 
                choices = unique(testDT$M),
        )       

        testDT1 <- reactive({
                    if (input$N == 0 & input$M == 0){
                        testDT
                    } else if(input$N != 0 & input$M == 0) {
                        testDT[eval(call("%in%", as.name("N"), input$N))]   
                    } else if(input$M != 0 & input$N == 0) {
                        testDT[eval(call("%in%", as.name("M"), input$M))]
                    } else {
                        testDT[eval(call("%in%", as.name("N"), input$N)) & 
                                        eval(call("%in%", as.name("M"), input$M))]                          
                    }
                })

        output$testTable <- renderDataTable(testDT1())

        output$testPlot1 <- renderPlot({

                    p <- ggplot(testDT1(), aes(L,O)) +
                            geom_boxplot(aes(fill = N)) +  
                            theme_bw() +
                            theme(legend.position = "top", legend.title=element_blank()) + 
                            facet_grid(M ~ ., scales = "free") +
                            labs(x = "L", y = "O")
                    print(p)
                })
    })

When I open the app, both N and M have empty sets. I am hoping to find a way that each selectizeInput box will have all options selected on open (AKA no subsetted dataset) but also still show the empty placeholder.

I created this function to conditionally display everything when there is nothing inputted into one or both of the selectizeInput boxes:

        testDT1 <- reactive({
                    if (input$N == 0 & input$M == 0){
                        testDT
                    } else if(input$N != 0 & input$M == 0) {
                        testDT[eval(call("%in%", as.name("N"), input$N))]   
                    } else if(input$M != 0 & input$N == 0) {
                        testDT[eval(call("%in%", as.name("M"), input$M))]
                    } else {
                        testDT[eval(call("%in%", as.name("N"), input$N)) & 
                                        eval(call("%in%", as.name("M"), input$M))]                          
                    }
                })

Unfortunately, it seems as if a input$N == 0 is not what an empty set of input$N produces. I've tried 0, NA, and "" but none of those work.

So the question is: What is the output for input$N when nothing is selected in the selectizeInput box?

Thanks in advance!

EDIT: Thanks to @Romain, I modified the function to produce this:

        testDT1 <- reactive({
                    if (is.null(input$N) & is.null(input$M)){
                        testDT
                    } else if(!is.null(input$N) & is.null(input$M)) {
                        testDT[eval(call("%in%", as.name("N"), input$N))]   
                    } else if(is.null(input$N) & !is.null(input$M)) {
                        testDT[eval(call("%in%", as.name("M"), input$M))]
                    } else {
                        testDT[eval(call("%in%", as.name("N"), input$N)) & 
                                        eval(call("%in%", as.name("M"), input$M))]                          
                    }
                })

Which works perfectly

Answer 1

When nothing is selected the input is NULL.

Use is.null() if you want to check if nothing is selected.