library(shiny)

ui <- fluidPage(
    shinyalert::useShinyalert(),
    actionButton('test', 'test')
)

server <- function(input, output, session) {
    observeEvent(input$test, {
        shinyalert::shinyalert("modal 1",
                               timer = 1000,
                               callbackR = function() {
                                   shinyalert::shinyalert("modal 2")
                               })
        
    })
}

shinyApp(ui, server)