Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 module

Last updated on
17 February 2018

This documentation is for modules. For the themes, see Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme.

In Drupal 8, stylesheets (CSS) and JavaScript (JS) are loaded through the same system for modules (code) and themes, for everything: asset libraries. Asset libraries can contain one or more CSS assets, one or more JS assets and one or more JS settings.

Drupal uses a high-level principle: assets (CSS or JS) are still only loaded if you tell Drupal it should load them. Drupal does not load all assets (CSS/JS) on all pages because this is bad for front-end performance.

Differences compared to Drupal 7

There are two important differences compared to Drupal 7 for developers:

  1. Only the JavaScript required on a particular page will be added to that page. In particular, by default Drupal doesn't need JavaScript on most pages that anonymous users can see. This means that jQuery is not automatically loaded on all pages anymore.
    So, if your theme requires jQuery or some other JavaScript to be present (which also is defined in an asset library), you need to tell Drupal that this is the case, by declaring a dependency on the needed asset library.
  2. The Drupal.settings  javascript object is replaced by drupalSettings.

The process

The general steps for loading assets (CSS/JS) are:

  1. Save the CSS or JS to a file.
  2. Define a "library", which can contain both CSS and JS files.
  3. "Attach" the library to a render array in a hook.

But in the case of themes, there is an alternative to step 3: themes can choose to load any number of asset libraries on all pages.

Defining a library

To define one or more (asset) libraries, add a *.libraries.yml file to the root of your module folder (alongside your .info.yml file). (If your module is named fluffiness, then the file name should be fluffiness.libraries.yml). Each "library" in the file is an entry detailing CSS and JS files (assets), like this:

  version: 1.x
      css/cuddly-slider-layout.css: {}
      css/cuddly-slider-theme.css: {}
    js/cuddly-slider.js: {}

You may notice the 'layout' and 'theme' keys for css which is not present for js. This indicates the style type the css file belongs to.

You can set CSS weights with 5 different levels of styling:

  • base: CSS reset/normalize plus HTML element styling. Key assigns a weight of CSS_BASE = -200
  • layout: macro arrangement of a web page, including any grid systems. Key assigns a weight of CSS_LAYOUT = -100
  • component: discrete, reusable UI elements. Key assigns a weight of CSS_COMPONENT = 0
  • state: styles that deal with client-side changes to components. Key assigns a weight of CSS_STATE = 100
  • theme: purely visual styling (“look-and-feel”) for a component. Key assigns a weight of CSS_THEME = 200

This is defined by the SMACSS standard. So here if you specify theme it means that the CSS file contains theme related styling which is pure look and feel. More info here. You cannot use other keys as these will cause strict warnings.

This example assumes that the actual JavaScript cuddly-slider.js is located in the subfolder js of your module. You can also have the JS come from an external URL, include CSS files, and there are other possibilities. See CDN / externally hosted libraries for details.

However, remember that Drupal 8 no longer loads jQuery on all pages by default; Drupal 8 only loads what's necessary. Therefore, we must declare that our module's cuddly-slider library declares a dependency on the library that contains jQuery. It is neither a module nor a theme that provides jQuery, it's Drupal core: core/jquery is the dependency we want to declare. (This is an extension name followed by a slash, followed by the library name, so if some other library wanted to depend on our cuddly-slider library, it'd have to declare a dependency on fluffiness/cuddly-slider, because fluffiness is the name of our module.)

So, to ensure jQuery is available for js/cuddly-slider.js, we update the above to:

  version: 1.x
      css/cuddly-slider.css: {}
    js/cuddly-slider.js: {}
    - core/jquery

As you'd expect, the order the CSS and JS assets are listed is also the order in which they will be loaded.

Attaching a library to page(s)

Depending on which assets you need to have loaded, you'll want to attach the corresponding asset library in a different way. After all, some asset libraries are needed on all pages, others only very rarely, and yet others on most, but not quite all.

But what matters most is that we don't decide whether to attach a library based on which page we're on (i.e. which URL or route), but based on which things are visible on the page: if a page contains a '#type' => 'table', a '#type' => 'dropbutton' and a '#type' => 'foobar', then we'll only load the libraries associated with each of those '#type's.
But we're not limited to '#type' only: perhaps we want to load a certain asset library only for a certain instance of a '#type'. In that case, we just attach it to the render array of that instance.

Of course, very rarely, there is a valid reason to actually load a certain asset on all pages (e.g. some analytics JavaScript that tracks page loads), regardless of the "things" on a page.

The sub-sections here show examples of how to do these things.

Attaching to a certain '#type' (for all instances of it)

To attach a library to a certain existing '#type', for all instances of it, we use hook_element_info_alter():

function yourmodule_element_info_alter(array &$types) {
  if (isset($types['table'])) {
    $types['table']['#attached']['library'][] = 'your_module/library_name';

Then clear the cache so that Drupal is aware of the new hook implementation you added.

Attaching to a render array

To attach a library to a render array (and perhaps a specific instance of a certain '#type'), you must have access to that render array. Perhaps you're defining the render array. Perhaps you're modifying it in a hook. In either case, it will look somewhat like this:

$build['the_element_that_needs_the_asset_library']['#attached']['library'][] = 'your_module/library_name';

Attaching to a render array of a Block Plugin

To give another example of attaching a library to a render array, If you are building a block plugin in your module, you can attach the libraries to the render array in the build() function of your class extending the BlockBase class (as of Drupal 8 beta 6).

    return [
      '#theme' => 'your_module_theme_id',
      '#someVariable' => $some_variable,
      '#attached' => array(
        'library' => array(

Attaching a library to all (or a subset of) pages

In some cases, the asset library is not associated with a certain part of the page, because it is associated with the entire page. For this case, hook_page_attachments() exists. A clear example can be found in the Contextual Links module:

// From core/modules/contextual/contextual.module.
function contextual_page_attachments(array &$page) {
  if (!\Drupal::currentUser()->hasPermission('access contextual links')) {

  $page['#attached']['library'][] = 'contextual/drupal.contextual-links';

Attaching a library in a preprocess function

You can attach a library in a preprocess function using the special key '#attached':

function yourmodule_preprocess_maintenance_page(&$variables) {
  $variables['#attached']['library'][] =  'your_module/library_name';

Attaching a library in a twig template

You can also attach a library in a twig template by using the attach_library() twig function. So in any *.html.twig:

{{ attach_library('your_module/library_name') }}
<div>Some markup {{ message }}</div>

Attaching configurable JavaScript

In some cases, you may want to add JavaScript to a page that depends on some computed PHP information.

In this case, create a JavaScript file, define and attach a library just like before, but also attach JavaScript settings and have that JavaScript file read those settings, via drupalSettings (the successor to Drupal 7's Drupal.settings). However, to make drupalSettings available to our JavaScript file, we have to do the same work as we had to do to make jQuery available: we have to declare a dependency on it.

So that then becomes:

  version: 1.x
    js/cuddly-slider.js: {}
    - core/jquery
    - core/drupalSettings


In your module call hook_page_attachments_alter(&$build), inside the function call:

$build['#attached']['library'][] = 'your_module/library_name';
$build['#attached']['drupalSettings']['fluffiness']['cuddlySlider']['foo'] = 'bar';

Where 'bar' is some calculated value.

Then cuddly-slider.js will be able to access (and it will === 'bar').

Adding attributes to script elements

If you want to add attributes on a script tag, you need to add an attributes key to the JSON following the script URL. Within the object following the attributes key, add the attribute name that you want to appear in the script as a new key. The value for this key will be the attribute value. If that value is set to true, the attribute will appear on its own without a value on the element.

For example: {type: external, attributes: { defer: true, async: true, data-test: map-link } }

This would result in the following markup:

<script src="" async defer data-test="map-link"></script>

Disabling Aggregation

By default, multiple local files will be aggregated where possible. To disable this for a file, set its 'preprocess' flag to false.

  version: 1.x
    js/cuddly-slider.js: {preprocess: false}
    - core/jquery
    - core/drupalSettings

CDN / externally hosted libraries

You might want to use JavaScript that is externally on a CDN (Content Delivery Network) to improve page loading speed. This can be done by declaring the library to be "external". It is also a good idea to include some information about the external library in the definition.

  version: 1.4.4
    name: MIT
    gpl-compatible: true
  js: { type: external, minified: true }

Inline JavaScript

Inline JavaScript is highly discouraged. It's recommended to put the JS you want to use inline in a file instead because that allows that JavaScript to be cached on the client side. It also allows JavaScript code to be reviewed and linted. Inline JS with also conflict with the Content Security Policy of many sites and make your module unusable by them.

Inline JavaScript that generates markup

This is discouraged. Place the javascript in a file instead. Examples of this are ads, social media sharing buttons, social media listing widgets. These do use inline JavaScript. But they are just a special kind of content/markup, since they're not about decorating the site's content or making it interactive, instead they are about pulling in external content through JavaScript.

You want to put these in either a custom block or even directly in a Twig template.


<script type="text/javascript"><!--
ad_client_id = "some identifier"
ad_width = 160;
ad_height = 90;
<script type="text/javascript" src=""></script>
<a class="twitter-timeline" href="" data-widget-id="307116909013368833">Tweets by @wimleers</a>
<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0],p=/^http:/.test(d.location)?'http':'https';if(!d.getElementById(id)){js=d.createElement(s);;js.src=p+"://";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs");</script>

Inline JavaScript that affects the entire page

Inline JavaScript is highly discouraged. Examples of inline JavaScript that affects the entire page are analytics (e.g. Google Analytics) and hosted font services. Inline JavaScript that affects the entire page can be in either of two categories: front-end/styling, or logical. Most of these cases can be satisfied with fixed javascript in a file plus added settings.

In the case of front-end/styling (e.g. hosted font services), it belongs in the theme, and for that, please see “Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme”.

In the other case, the JS belongs in the module. In the appropriate hook — likely hook_page_attachments() — define attached HTML <HEAD> data by using the 'html_head' key in the #attached property:

function fluffiness_page_attachments(array &$attachments) {
  $attachments['#attached']['html_head'][] = [
    // The data.
      '#type' => 'html_tag',
      // The HTML tag to add, in this case a  tag.
      '#tag' => 'script',
      // The value of the HTML tag, here we want to end up with
      // alert("Hello world!");.
      '#value' => 'alert("Hello world!");',
      // Set attributes like src to load a file.
      '#attributes' => array('src' => ''),

    // A key, to make it possible to recognize this HTML  element when altering.

Dynamically generated CSS and JS

In extremely rare and advanced cases, you may have the need to dynamically generate CSS and JS. There are two categories of "dynamicness":

  1. Dynamically built, but used across multiple requests
  2. Dynamically built for each request

If the dynamic CSS/JS is used across multiple requests, then you can use hook_library_info_alter() to modify a library to include your dynamically/automatically generated CSS/JS. An example in Drupal 8 core of this is color_library_info_alter(). Realize that just using hook_library_info_build() or hook_library_info_alter() to append a library will not automatically make the library appear in the page. You still have to define it as an attachment (either for the page or for a certain element) by using any of the techniques above. 

If the dynamic CSS/JS is built for each request, then you enter the truly advanced territory. This is hard, and for a good reason: per-request dynamic assets have to be built on every single request and therefore slow Drupal down. We want to make it hard to make Drupal slow down, so this is why we don't offer a nice API for this — since we don't want you to do it.
It is possible though. In the case of dynamic JS: please consider using configurable JavaScript instead, that is almost always the much better choice. Then the logic is stored in a file (and can be reviewed, linted and cached on the client side), and only the settings to configure the logic in that file need to be generated on each request. And in fact, this can also be used for dynamic CSS: attach dynamic CSS as drupalSettings and let some JS file add it to the page.
If using drupalSettings plus a JavaScript file is not an option, then you still have one option left: use hook_page_attachments(), where you add a new value to $page['#attached']['html_head'], which contains either a <script> tag or a <style> tag, as the “Inline JavaScript that affects the entire page” section above already showed.

hook_library_info_build() added for dynamic library definitions

For some advanced use cases — like detecting 3rd party libraries that need to be downloaded manually, and then exposing those as Drupal asset libraries (think Libraries API module) — you want to be able to still use PHP code to register libraries using some additional logic. That's why hook_library_info_build() was added

Note that "dynamic" doesn't mean "runtime" (i.e. for every request) — that'd be terrible for performance. The dynamically added libraries are still cached, just like libraries defined in YML files. This means that you still need to attach the library to a page or element using any of the above techniques. It's "dynamic" because you can use logic to control this attaching of the libraries.

Differences with Drupal 7

More information