adding_new_css_and_js.diviner

@title Adding New CSS and JS
@group developer

Explains how to add new CSS and JS files to Phabricator.

= Overview =

Phabricator uses a system called **Celerity** to manage static resources. If you
are a current or former Facebook employee, Celerity is based on the Haste system
used at Facebook and generally behaves similarly.

This document is intended for Phabricator developers and contributors. This
process will not work correctly for third-party code, plugins, or extensions.

= Adding a New File =

To add a new CSS or JS file, create it in an appropriate location in
`webroot/rsrc/css/` or `webroot/rsrc/js/` inside your `phabricator/`
directory.

Each file must ##@provides## itself as a component, declared in a header
comment:

  LANG=css
  /**
   * @provides duck-styles-css
   */

  .duck-header {
    font-size: 9001px;
  }

Note that this comment must be a Javadoc-style comment, not just any comment.

If your component depends on other components (which is common in JS but
rare and inadvisable in CSS), declare then with `@requires`:

  LANG=js
  /**
   * @requires javelin-stratcom
   * @provides duck
   */

  /**
   * Put class documentation here, NOT in the header block.
   */
  JX.install('Duck', {
    ...
  });

Then rebuild the Celerity map (see the next section).

= Changing an Existing File =

When you add, move or remove a file, or change the contents of existing JS or
CSS file, you should rebuild the Celerity map:

  phabricator/ $ ./bin/celerity map

If you've only changed file content things will generally work even if you
don't, but they might start not working as well in the future if you skip this
step.

The generated file `resources/celerity/map.php` causes merge conflicts
quite often. They can be resolved by running the Celerity mapper. You can
automate this process by running:

  phabricator/ $ ./scripts/celerity/install_merge.sh

This will install Git merge driver which will run when a conflict in this file
occurs.

= Including a File =

To include a CSS or JS file in a page, use
@{function:require_celerity_resource}:

  require_celerity_resource('duck-style-css');
  require_celerity_resource('duck');

If your map is up to date, the resource should now be included correctly when
the page is rendered.

You should place this call as close to the code which actually uses the resource
as possible, i.e. **not** at the top of your Controller. The idea is that you
should @{function:require_celerity_resource} a resource only if you are actually
using it on a specific rendering of the page, not just because some views of the
page might require it.

= Next Steps =

Continue by:

  - reading about Javascript-specific guidelines in @{article:Javascript Coding
    Standards}; or
  - reading about CSS-specific guidelines and features in @{article:CSS Coding
    Standards}.