# 02. Day to day development

#### Colin Fay

#### 2026-06-25

## Day to day development with `{golem}`

Now that you’re all set with your project init, time to move to development!

App development should happen through the `dev/02_dev.R` file, which contains common commands for developing.

## Launching the app

To run the app, go to the `dev/run_dev.R` file, and run the all thing.

## `dev/02_dev.R`

### Add dependencies

You can use `attachment::att_amend_desc()` to parse the code in your `.R` files, detect `@import` `@importFrom` and `::`, and fill your `DESCRIPTION` accordingly.

Note that the `{attachment}` package should be installed on your machine.

    attachment::att_amend_desc()

About package dependencies.

### Add modules

The `golem::add_module()` functions creates a module in the `R` folder. The file and the modules will be named after the `name` parameter, by adding `mod_` to the R file, and `mod_*_ui` and `mod_*_server` to the UI and server functions.

    golem::add_module(name = "my_first_module") # Name of the module

The new file will contain:

    # mod_UI
    mod_my_first_module_ui <- function(id) {
      ns <- NS(id)
      tagList()
    }

    mod_my_first_module_server <- function(input, output, session) {
      ns <- session$ns
    }

    ## To be copied in the UI
    # mod_my_first_module_ui("my_first_module_1")

    ## To be copied in the server
    # callModule(mod_my_first_module_server, "my_first_module_1")

At the end of the file, you will find a piece of code that has to be copied and pasted inside your UI and server functions.

## Add function files

    golem::add_fct("helpers")
    golem::add_utils("helpers")

These two function create `R/fct_helpers.R` and `R/utils_helpers.R`, two file you can use to add business logic functions.

### Add external files

These functions create external dependencies (JavaScript, CSS and Sass). `add_js_file()` creates a simple JavaScript file, while `add_js_handler()` adds a file with a skeleton for shiny custom handlers.

    golem::add_js_file("script")
    golem::add_js_handler("script")
    golem::add_css_file("custom")
    golem::add_sass_file("custom")

Note: While the general philosophy of `{golem}` is being based on the idea that you’re building a package, these functions can be used outside of a `{golem}` project.

Note that you can also download external CSS and JavaScript files with:

    golem::use_external_css_file(url = "url", name = "your_provided_name")
    golem::use_external_js_file(url = "url", name = "your_provided_name")

The above has the effect of downloading the file at the specified url and giving it a provided name. If the intent is to use a CDN hosted CSS/JS file then manually add the tag - `tags$script(src = "source_of_ur_css/js")` in the function `golem_add_external_resources` in file `app-ui.R`. The tag can be added inside the `tags$head(...)` if the intent is to load the js/css file in the `<head>` of the document or outside it if it is intended in the `<body>`.

### Adding these external resources to your app

You can add any external resource into `inst/app/www`.

JavaScript and CSS are automatically linked in the `golem_add_external_resources()` function. If you add other resources (example images), you can link them in the app with the `www` prefix:

    tags$img(src = "www/my.png")

You can also list here the use of other packages, for example `useShinyalert()` from the `{shinyalert}` package.

### Add a data-raw folder

If you have data in your package:

    usethis::use_data_raw()

About data in a package.

### Add tests

Add more tests to your application:

    usethis::use_test("app")

About testing a package.

## Documentation

### Vignette

    usethis::use_vignette("shinyexample")
    devtools::build_vignettes()

About package Vignette.

### Code coverage

    usethis::use_coverage()

## Using `{golem}` dev functions

There’s a series of tools to make your app behave differently whether it’s in dev or prod mode. Notably, the `app_prod()` and `app_dev()` function tests for `options( "golem.app.prod")` (or return TRUE if this option doesn’t exist).

Setting this options at the beginning of your dev process allows to make your app behave in a specific way when you are in dev mode. For example, printing message to the console with `cat_dev()`.

    options("golem.app.prod" = TRUE)
    golem::cat_dev("hey\n")
    options("golem.app.prod" = FALSE)
    golem::cat_dev("hey\n")
    #> hey

You can then make any function being “dev-dependent” with the `make_dev()` function:

    log_dev <- golem::make_dev(log)
    log_dev(10)
    #> [1] 2.302585
    options("golem.app.prod" = TRUE)
    log_dev(10)