To load custom JS or CSS in a Shopify theme, upload the file to the assets folder and reference it in theme.liquid with the asset_url plus script_tag or stylesheet_tag filters. Use the .liquid extension on the asset name when you need Liquid tags evaluated inside the file.

Context: Stack Overflow #20467240 (accepted answer, 46 votes): the asker uploaded JS and CSS files as theme assets but did not know how to reference them on a specific page. The accepted answer (46 votes) gives the canonical pattern: drop [filename].js.liquid into theme.liquid's head section with the asset_url and script_tag filters. Naming the file with a .liquid extension also lets Liquid evaluate inside the asset when needed.