In R, under options(), some settings regarding the use of R in general can be seen, for example, the number of digits printed for a number. These options can be modified by typing options (option_name = value), for example:

options(digits = 15)

Shiny has several options that are exclusive to the library. Almost all of them are defaulted to NULL. However, they can be changed in the same way as any other R option. Probably, the most important ones among them are shiny.trace and shiny.error. For shiny.trace, if TRUE, all of the messages sent between the R server and the web browser client will be printed on the console. This is useful for debugging (http://shiny.rstudio.com/reference/shiny/latest/shiny-options.html). For shiny.error, this can be a function that is called when an error occurs. For example, the option (shiny.error=recover) will result in the debugger prompt when an error occurs. The full list can be seen at http://shiny.rstudio.com/reference/shiny/latest/shiny-options.html.