Code Snippets for JS and CSS Assets on Drupal 8

JS and CSS assests in drupal are highly customizable. For more information, refer to this blog post.

Here are some practical scenarios:

Through a global library
Deleting or overriding existing styles served by core
Declaring various dependencies that are provided by Drupal 8's core
Specifying assets scope or sending JS to the footer
Specifying media type
Adding external CSS or JS library, e.g: Google Fonts
Adding conditional CSS or JS for older IE browsers
Adding custom libraries to specific pages, e.g: front page

Global styles and scripts

In the example below global-styling is a name of a library (a file or collection of files) that is used for global styles. It is declared in a theme_name.libraries.yml and then also added to the theme_name.info.yml file. This specific library will now be available across the whole site.

Here is how it will look in the theme_name.libraries.yml file.

  
    global-styling:
      version: VERSION
      css:
        theme:
          css/style.css: {}

Then it's declared in the theme_name.info.yml file.

 
   name: Example
   type: theme
   core: 8.x
   libraries:
   - theme_name/global-styling

Declaring scripts would be very similar. In the theme_name.libraries.yml

  
    global-scripts:
      version: VERSION
      js:
        js/script.js: {}

Then in the theme_name.info.yml file, we add these libraries.

  
    name: Example
    type: theme
    core: 8.x
    libraries:
    - theme_name/global-styling
    - theme_name/global-scripts

Now these two libraries or collections of JS and CSS files are available across the whole site. These two separate sets of CSS and JS can also be combined into one library.

  
    global-styles-and-scripts:
      version: VERSION
      css:
        theme:
           css/style.css: {}
      js:
        js/script.js: {}

Deleting or overriding existing styles served by core

In the theme_name.info.yml file:

  
    stylesheets-remove:
      - normalize.css

or to override the core's stylesheet:

  
    stylesheets-override:
      - normalize.css

Declaring dependencies

Example of declaring a dependency.

  
    global-scripts:
      version: VERSION
      js:
        js/script.js: {}
      dependencies:
        - core/jquery
        - core/jquery.once
        - core/modernizr

Specifying scope of a JS file or sending it to the footer

There are many cases when it would be best to have a JS asset at the footer of the page. By default they will be included in the header but you can easily send them to the footer by specifying the scope in the theme_name.libraries.yml file.

  
    js/script.js: { scope: footer }

Specifying media type

Similarly, if there is a need for a print stylesheet in your theme it can also be easily specified with a media parameter in the theme_name.libraries.yml file.

  
    css/print.css: { media: print }

Bringing in an external JS or CSS resource

Often you would want to include an external library in your theme, here is how it can be added in theme_name.libraries.yml file.

  
    remote-custom:
      remote: https://github.com/yui/yui3
      license:
        name: BSD
        url: https://github.com/yui/yui3/blob/master/LICENSE.md
        gpl-compatible: true
      version: VERSION
      js:
        http://yui.yahooapis.com/3.18.1/build/yui/yui-min.js:  { type: external, browsers: { IE: 'lte IE 8', '!IE': false } }

You can also specify additional parameters in the theme_name.theme file with a preprocess hook:

  
        function theme_name_css_alter(&$css) {
        $theme_path = drupal_get_path('theme', 'theme_name');
        // Add googlefonts.
        $googlefonts = '//fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,700|Source+Serif+Pro:700,400';
        $css[$googlefonts] = array(
        'data' => $googlefonts,
        'type' => 'external',
        'every_page' => TRUE,
        'media' => 'all',
        'preprocess' => FALSE,
        'group' => CSS_AGGREGATE_THEME,
        'browsers' => array('IE' => TRUE, '!IE' => TRUE),
        'weight' => -1,
        );
      }

Conditional CSS or JS for older IE browsers

You can specify IE conditionals through the parameters in the theme_name.libraries.yml file.

  
    http://yui.yahooapis.com/3.18.1/build/yui/yui-min.js:  { type: external, browsers: { IE: 'lte IE 8', '!IE': false } }

Adding or limiting a library to specific pages

If you only need to load a specific library onto a specific page, e.g maintenance page, you will need to add a hook in your theme_name.theme file. Here is an example:

  
    function theme_name_preprocess_maintenance_page(&$variables) {
      $variables['#attached']['library'][] = 'theme_name/custom-library';
    }

Here is an example of how a custom-library can be loaded only on a front page:

  
    function theme_name_preprocess_page(&$variables) {
        if($variables['is_front']){
          $variables['#attached']['library'][] = 'theme_name/custom-library';
      }
    }

Remember to always clear your caches when you are adding libraries or creating new hooks. An easy way to do that automatically is to set the debug: true in your Drupal 8 services.yaml file in sites/default/ while developing.