Loading and Using JavaScript in Drupal 8

Drupal 8 now exclusively uses a “libraries” concept.

For more info on JS in D8, check out my JavaScript related changes in Drupal 8.4, and beyond

Update 10/22/17 — updates after giving Using JavaScript in D8, and Intro to ES6 (new in 8.4 Core), I’ve tweaked some examples and included some info on core libraries at the end.

Update 2/21/17 — after presenting this as part of my Using JavaScript in D8, and an intro to the bundled JS Libraries session, I received some feedback on how Protocol-relative URLs are considered an anti-pattern, using hook_page_attachments over theme_preprocess_hook for modules, and I added a comment on PSR-4 namespacing.

Drupal 8’s JavaScript handling is a conceptual extension of D7’s hook_library()and the D7 Libraries API Module.

Libraries are defined in YAML format, of course. You can define one or more libraries in each file.

Libraries are also used for CSS but we’ll ignore that for this discussion.

Define a Library

Create or edit the <some-file-name>.libraries.yml file that goes in the root of your theme or module:

js-path/my-theme.js: {}

some-file-name can be any name really, but should be the name of your theme or module.

Drupal will aggregate JS files by default, so to avoid that if necessary:

js-path/my-theme.js: { preprocess: false }

Of course, adjust js-path as needed. See PSR-4 namespaces and autoloading in Drupal 8 for more info on pathing.

Library Options

You can optionally provide a version number per library, but Drupal doesn’t use it for anything:

version: "1.0.1"

You can also define dependencies per library, which are followed:

- core/jquery

D8 by default loads JS into the footer; to load to the header:

header: true

Attaching Libraries in a Theme

Reference them in your my-theme-name.info.yml file. They load globally as in on every page.

- core/jquery
- my-theme-name/this-lib-name

Here we’re loading our libraries this-lib-name and core/jquery (which is no longer loaded by default).

Attaching Libraries in Twig

You can attach a JS library from a twig template. It will only load if the template loads which means you can conditionally load it based on the naming structure of the Twig template file.

{{ attach_library('my-theme-name/some-other-lib') }}

Attaching Libraries in your Module’s PHP

PHP allows you to control what libraries are loaded and when.

function mymodule_page_attachments(array &$attachments) {
$attachments['#attached']['library'][] =

Of course you can load any number of libraries using this, and related, hooks.

Attaching Libraries in your Theme’s PHP

If not loading everywhere by using the .yml file.

function mytheme_preprocess_page(&$variables) {
$variables['#attached']['library'][] =

See also related THEME_preprocess_HOOK()’s. Although some discourage using these hooks as they’re intended for preprocessing variables.

Conditional Libraries in PHP

To support D8’s caching, if you want to *conditionally* attach libraries, you need to provide cacheability metadata*.

function mytheme_or_module_preprocess_page(&$variables) {
$variables['page']['#cache']['contexts'][] = 'url.path';
// above line sets the cacheability metadata

if (\Drupal::service('path.matcher')->isFrontPage()) {
$variables['#attached']['library'][] =

*Metadata is comprised of a combination of up to 3 things: tags, contexts, and max-age. See the Cache API https://www.drupal.org/developing/api/8/cache for more info.

Loading External JavaScript

External “libraries” still have to be defined inside Drupal libraries*.

angular.js: { type: external }

Since Drupal will minify, if it already is, let D know:

{ type: external, minified: true }

If attributes are needed in the resulting <script> line:

{ type: external , attributes: { defer: true, async: true } }

*I “broke” the code lines for readability, but it’s not legal.

JavaScript Settings

To add “computed” settings or configuration, first you have to define a dependency on core/drupalSettings.

- core/drupalSettings


function mytheme_preprocess_page(&$vars) {
$vars['#attached']['library'][] = 'mytheme/this-lib';
['this-lib']['some-prop'] = $some_var;

Then access the some-prop setting in JS with:


Manipulating Libraries

The theme is able to use two new directives, libraries-extend and libraries-override to extend, remove or replace whole libraries or individual library files.

Both of these are placed in the theme’s my-theme.info.yml file.


By “extending” a library it means the new library is always loaded with the target library, regardless of conditions.

- mytheme/some-other-lib

Here, the some-other-lib of the theme is used to extend some-lib of some mytheme_or_module.


This directive is more powerful, but when dealing with JS (unlike CSS which may just result in poor layout/design), you can break JavaScript on your site by not paying close attention to JS code dependencies.

You can remove a whole library:

other_theme_or_module/some-lib: false

Or remove a library file:

full-path-to-library/some-file.js: false

Or replace a whole libs:

other_theme_or_module/some-lib: mytheme/some-other-lib

Or replace a library file:

full-path-to-library/some-file.js: path-to-theme-lib/some-other-file.js

Inline JavaScript

Almost always JavaScript should be included in a library.

But if you must, put the complete script tag in the html.html.twig file (yes, “html” is repeated twice.)

Dynamically Altering Libraries

Use this when you want to dynamically specify JS (or CSS) Libraries used across multiple requests.

hook_library_info_alter(&$libraries, $extension)

Allows modules and themes to change libraries' definitions.

An example of this is the Color module which uses this hook to replace CSS Libraries with the “colored” CSS libraries.

Dynamically Adding Libraries

This is most like D7’s Libraries. Use it to programmatically add an entire library definition. And they are still cached.

Modules may implement hook_library_info_build() to add dynamic library definitions
function my_module_library_info_build() {
$libs = array();
$libs['my-lib'] = [
'js' => [
'my-lib-code.js' => [],

Sneaky JavaScript

While you’re suppose to use libraries you could attach into HTML head and create a <script> tag directly. These are (re)built on every single request and therefore aren’t cached and can slow down Drupal, so beware.

If possible, it’s best to leave your JS file(s) static and just use JS Settings configurations.

$page['#attached']['html_head'][] = [
'#type' => 'html_tag',
'#tag' => 'script',
'#attributes' => [
'src' => 'some-path/my-file.js',
], 'my-mode-js', // an identifier for Drupal

Core Libraries

Core libs defined: core/core.libraries.yml. Examples, all prefixed with core\:

  • drupal
  • drupalSettings
  • drupal.autocomplete*
  • drupal.debounce
  • drupal.tabledrag
  • jquery
  • jquery.joyride
  • jquery.ui
  • jquery.ui.autocomplete*
  • underscore

* Note that many jQuery libs are “wrapped” by Drupal libs. Best to use the Drupal lib.

In Closing

Hopefully this shed some light on D8’s JavaScript handling. Happy coding.