Shopify App Script Checker

Where Shopify storefront JavaScript comes from

Every script on a storefront page was put there by one of four parties, and the path it loads from gives the party away. That matters because each group has a different owner, and the owner decides whether you can remove it.

• The theme. Files under /cdn/shop/t/<id>/assets/ and inline blocks written in Liquid. Edited in the theme code editor. Includes any tag a developer pasted into theme.liquid, which is where old app code hides.
• Shopify itself. Checkout preloads, Shop Pay, the web pixels manager, analytics and the performance kit, served from /cdn/shopifycloud/ and /cdn/wpm/ and written by content_for_header. Present on every store.
• Theme app extensions. App blocks and app embed blocks, with assets hosted on Shopify's CDN under /extensions/. Shopify renders app embeds before the closing head and body tags and wraps each in a comment naming the app.
• The legacy ScriptTag API. Remote URLs an app registered, which Shopify injects on window load. Theme app extensions replaced it in 2021 and Shopify stops running storefront script tags on March 1, 2027.

Third-party hosts cut across all four. An app embed is usually a small loader that fetches the vendor's real bundle from its own CDN, and a tag manager fans out to a dozen more. The checker lists every third-party host in the HTML and, separately, the hosts inline code names but no tag loads.

Why app scripts dominate Core Web Vitals on Shopify stores

Google's thresholds for a good Core Web Vitals result, measured at the 75th percentile of real visits, are an LCP of 2.5 seconds or less, an INP of 200 milliseconds or less, and a CLS of 0.1 or less. Shopify's own performance guidance names JavaScript as the primary cause of poor INP and delayed LCP.

Shopify's help centre lists the three biggest factors in store performance as the theme, the installed apps, and third-party code added by hand, such as tag managers. The hosting, the CDN and image compression are Shopify's problem and are already handled. The scripts are yours.

How a script hurts each metric

• LCP. A parser-blocking script in the head delays every element below it, including the product image. Shopify's first JavaScript recommendation is to remove render-blocking apps.
• INP. Every kilobyte that executes on the main thread competes with the tap on Add to cart. Five review, upsell and chat widgets each doing a little add up to a page that answers late.
• CLS. App-injected content that arrives after paint, such as a banner or a reviews strip, pushes the page down. Shopify names app-injected content as a common cause.

Google's guidance on third-party JavaScript is short: load it with async or defer, self-host it if the vendor is slow, and remove it if it does not add clear value. This inventory is what lets you apply the third rule. The Shopify speed optimization guide covers the rest of the page.

How to read the report

The report answers one question per block: how much, from whom, and loaded how. Start at the group table and work down. The table below is the order to cut in, with what each group is and how it comes out.

Loading flags: async, defer and blocking

An external script with neither async nor defer is marked blocking: the browser stops building the page until that file has downloaded and run. Modules defer by default. Shopify's guidance is defer for anything non-critical and async only for independent third-party scripts where order does not matter.

The five heaviest table combines sized external files with the length of each inline block. A 50 KB inline block from an app embed costs parse time on every visit exactly like an external file would, and it never caches.

Apps that left code behind

Uninstalling an app removes its embeds and its ScriptTags, but not code it wrote into theme.liquid or a snippet. In the report that shows up as a third-party host with no entry in the apps-identified list beside it. Search the theme code for the hostname, and add the check to your Shopify SEO audit.

What to do next

Six steps take a report to a lighter page. The order is deliberate: the two groups you control come first, the loading flags second, and the deprecation clean-up last because it has a date attached.

1. 01

   Pick a product page, not the homepage

   Product and collection templates carry the reviews, upsell, size-chart and wishlist blocks that homepages skip, and they are the pages that rank. Paste the full URL including https:// and run the check. One URL per run; the report reads that page's HTML only.

2. 02

   Read the four group counts first

   Third-party origins and app scripts are the two you can change. Theme assets you can edit but usually need. Platform scripts are Shopify's own checkout, analytics and pixel code, identical on every store, and the report lists them so you stop trying to remove them.

3. 03

   Match each third-party host to an app or a person

   Every host in the third-party list came from somewhere: an app embed, a tag pasted into theme.liquid, or a tag manager. The apps-identified list names the app embeds. A host with no app beside it is either a manual tag or a leftover from an uninstalled app.

4. 04

   Check the loading flags on the five heaviest

   A large script marked blocking has neither async nor defer, so the browser stops parsing the page until it downloads. That is the single attribute most likely to move LCP. Ask the app vendor for a deferred embed, or move a manual tag to the end of the body with defer.

5. 05

   Turn off what the page does not use

   In Online Store, Themes, Customize, open App embeds and switch off any embed the page does not need. Uninstall apps you no longer use rather than just closing them. Then rerun the check: the app group count and the third-party host list should both fall.

6. 06

   Migrate anything in the legacy ScriptTag list

   Scripts under Legacy ScriptTag API are injected by apps using an API Shopify is removing. Storefront script tags stop running on March 1, 2027. Ask each vendor whether their app has moved to a theme app extension, and replace the ones that have not.

Before adding anything back, check the app against the Shopify SEO apps roundup, which notes what each one loads on the storefront, and keep the script count on your Shopify SEO checklist so it is rechecked after every app install. The page speed entry explains how the metrics relate to what you removed.

What this tool reports

Frequently asked questions