Adding custom CSS and JS to Shopify

If I understand correctly, the asset_url filter is what you are looking for.

To include a JS file in a .liquid file, you use:

{{ 'script.js' | asset_url | script_tag }}

And a CSS file:

{{ 'style.css' | asset_url | stylesheet_tag }}

If you don't want to pollute the global namespace you can restrict the JavaScript or CSS to certain areas.

Shopify uses a simple if statement together with the page-handle for that(for www.foo.com/abcd/bar - "bar" would be the handle).

{% if page.handle == "bar" %}
    {{ 'your_custom_javascript.js' | asset_url | script_tag }}
    {{ 'your_custom_css.css' | asset_url | stylesheet_tag }}
{% endif %} 

This is extremely useful if you want to make smaller changes to certain pages.

Simply upload your filename.js file in the assets folder.

then drop the following in your theme.liquid head section:

{{ 'filename.js' | asset_url | script_tag }}

by the way, you should rename your file and add .liquid extension

filename.js.liquid

Good luck!