Foremost in your mind should be the quintessential reality of R: Everything that happens in R is the result of a function call. Shiny is no exception.

To write a minimal shiny app, you create an object that describes your app’s user interface, write a function describing runtime behaviors, and pass them into a function that spins up the app you’ve described. By convention, these two objects are associated with the variable names ui and server.

library(shiny)
ui <- fluidPage()
server <- function(input, output, session) {}

This is just R code. You can type it into the Console to execute it line by line and inspect what it does.

If you’re working in RStudio, you can type it into a Source file, then press Control-Enter (Windows) or Command-Return (MacOS) to send each line to the Console for execution.

Checking the Environment—or the structure of these two objects with str()—we can see that ui is a list of three objects. If we print ui to the Console, we see only an empty HTML <div> element.

<div class="container-fluid"></div>

The object associated with server is simply a function with no body.

To execute this minimal shiny app, we pass the ui and server objects to the shinyApp() function.

shinyApp(ui, server)

The app will be spun up either in RStudio’s Viewer pane, in a Viewer window, or in your default Web browser, depending on your settings in RStudio.

Don’t be surprised: it will be just a blank window, since all that has been defined thus far is an empty <div> element. The document that opened is an HTML document with some boilerplate CSS and JavaScript. You can inspect it using your Browser’s Developer Tools.

That’s it. That’s shiny. Everything else flows from these core ideas:

• ui is a list object representing the HTML UI to be constructed.
• server is a function describing the runtime behavior of your app.
• shinyApp() takes these two objects and uses them to construct an HTML document that then gets spun up in a browser.