# Using Javascript

Unless there’s a good reason to put them elsewhere, scripts should live in assets/js, and be included in bundle.js, and not added separately. This is especially important in epubs. (More on scripts in epubs below.

If you must add scripts that are not included in bundle.js, add them to your <head> elements by adding <script> tags to _includes/head-elements. And to add them just before the </body> tag, add <script> tags to _includes/end-elements.

For web outputs, you can also link to remote scripts this way, without the need to save the actual scripts in the js folder.

Keep in mind that anything you add to head-elements will be added to all books in the project folder, and to all their formats.

## Limiting scripts by book or format

To limit a script to a given book or format, wrap the script tag in Liquid control flow tags. For instance:

{% include metadata %}
{% if book-directory == "grapes-of-wrath" %}
<script src="{{ site.baseurl }}/assets/js/grapes-of-wrath.js"></script>
{% endif %}


(The include metadata tag fetches all kinds of information about the page, including which book-directory it’s in.)

To limit a script to a given output format, use site.output:

{% if site.output == "print-pdf" %}

All scripts to be used in your epub should be included in assets/js/bundle.js, and not added to pages as separate files. This is because you must not have any scripts in your epub that you aren’t using, or it won’t validate; and we only support including bundle.js in epub output.
If you don’t want any Javsacript at all in your epub, you can disable it in _data/settings.yml by setting epub > javascript > enabled to false.You should then also exclude assets/js/bundle.js in the exclude list in _configs/_config.epub.yml.