Javascript in Shiny app only loads once - shiny

Question

Edit: Generalized question by finding reproducible example
The app below has two tabPanels, each with some Javascript within a sidebarPanel. I expect the Javascript in Tab 1 to run anytime Tab 1 is activated (i.e., when I initially launch the app and when I navigate back to Tab 1 from Tab 2). Also, I expect the Javascript in Tab 2 to run anytime I navigate to that tab.
Instead, the Javascript for both tabs runs immediately upon launching the app and then never again.
As context, I constructed this example because I ran into this problem while trying to use Javascript to place an Amazon Associates ad in my Shiny app.
Reproducible example
library(shiny)
ui = navbarPage( "",
tabPanel( "Tab 1",
mainPanel(
wellPanel("Blah blah blah"),
width = 6
),
sidebarPanel(
# only runs once, like the ads
HTML('<script type="text/javascript"> alert("Tab 1 is talking"); </script>')
# # runs every time
#HTML('<b> test </b>')
, width=6 )
),
tabPanel( "Tab 2",
mainPanel(
wellPanel("Blah blah blah"),
width = 6
), # ends mainPanel
sidebarPanel(
# only runs once, like the ads
HTML('<script type="text/javascript"> alert("Tab 2 is talking"); </script>')
# # runs every time
#HTML('<b> test </b>')
, width=6 )
)
)
server <- function(input, output) {
}
app = shinyApp( ui, server )

Answer 1

Tabs only toggle visibility of content already on the page. So when the app launches, Tab 2 is actually loaded but hidden. If you want tabs to dynamically add and remove scripts, you could use renderUI to do this based on the active tab. Or sendCustomMessage and addCustomMessageHandler.
Here's an example using renderUI:
library(shiny)
ui = navbarPage("", id = "navbar",
tabPanel( "Tab 1",
mainPanel(
wellPanel("Blah blah blah"),
width = 6
),
sidebarPanel(
uiOutput("tab1"),
width = 6
)
),
tabPanel( "Tab 2",
mainPanel(
wellPanel("Blah blah blah"),
width = 6
),
sidebarPanel(
uiOutput("tab2"),
width=6
)
)
)
server <- function(input, output) {
output$tab1 <- renderUI({
req(input$navbar == "Tab 1")
HTML('<script type="text/javascript"> alert("Tab 1 is talking"); </script>')
})
output$tab2 <- renderUI({
req(input$navbar == "Tab 2")
HTML('<script type="text/javascript"> alert("Tab 2 is talking"); </script>')
})
}
shinyApp( ui, server )