Over 94% of ecommerce sites are running five or more unresolved JavaScript errors right now. Most store owners have no idea. Here's what's breaking, why it's getting worse, and how to find it before it costs you another day of lost sales.

Broken JavaScript Is Killing Your Shopify Checkout (And You Probably Don't Know It)

Your Shopify store looks fine in Chrome on your MacBook. The product pages load, the cart opens, the checkout button works. You close the tab satisfied.

Meanwhile, a customer on a mid-range Android device in a patchy 4G signal hits your store, a third-party upsell widget throws an Uncaught TypeError, the add-to-cart button stops responding, and they leave. No error report. No alert. No indication anything went wrong on your end.

This is not an edge case. According to Noibu, over 94% of ecommerce websites have five or more unresolved JavaScript errors live at any given time. The vast majority of those errors never surface anywhere an owner or developer would see them, because only around 10% of affected users bother to report a bug. The other 90% just abandon.

What a silent JS error actually costs you

Cart abandonment is chronic across ecommerce, but the causes are not all equal. Shopify's own enterprise blog puts the overall abandonment rate at nearly 72%, with up to 18% of shoppers leaving specifically because checkout felt slow or broken, and another 15% leaving the moment a page crashes.

On a store doing 500 transactions a month with a £120 AOV, if 15% of checkout visitors are hitting a crash-level error, you're not looking at a minor annoyance. You're looking at a material chunk of potential revenue that disappears without a trace. No refund request, no complaint ticket, nothing.

The reason this stays invisible is structural. JavaScript errors happen client-side, inside the user's browser. They don't write to your server logs. Shopify's native analytics will show you a drop in checkout completion rate, but it won't tell you why. You're left guessing whether the problem is UX, pricing, or something actually broken.

Where the errors come from

The average Shopify store runs somewhere between 15 and 25 third-party scripts. Review apps, loyalty widgets, live chat, currency converters, upsell popups, heat mapping tools, affiliate trackers, they all inject JavaScript into your storefront, and they don't always play nicely together.

Invensis documents a pattern that will be familiar to any Shopify developer: conflicting JavaScript functions from upsell widgets, live chat popups, and currency converters frequently override one another or interfere with theme behaviour. One script redefines a global function that another depends on. A widget initialises before the DOM is ready and throws a null reference. An async request fails silently and leaves the cart in an inconsistent state.

These are not obscure bugs. They are the predictable result of loading 20 independent scripts written by 20 different teams onto the same page, none of whom know what the others are doing.

Unhandled promise rejections are a particularly common culprit. Shopify's own developer documentation explicitly recommends adding unhandledrejection listeners for promise rejections and error listeners for runtime exceptions and failed resource loads inside checkout UI extensions. The fact that they have to recommend it tells you how often it gets skipped.

August 2025 made this significantly worse

Shopify deprecated checkout.liquid, Additional Scripts, and legacy script tags in August 2025. The intention was to improve checkout speed and reduce the risk of broken or insecure code, as Digital Position outlined before the deadline. The reality for stores that haven't fully migrated is messier.

Revize documents the specific breakage happening right now: pixels and analytics tracking on Thank You and Order Status pages are failing, Shopify Scripts not migrated to Functions have stopped running, and third-party checkout customisations that relied on direct Liquid access no longer work. All of this is JavaScript-adjacent breakage that produces no visible error on screen, just silent data loss and silent conversion failure.

If your post-purchase pixel isn't firing, your Facebook ROAS reporting is wrong. If your checkout customisation broke silently, customers might be seeing a default experience that doesn't match what your brand promised. If your thank-you page script stopped running, your loyalty points aren't crediting. None of this shows up as a 500 error. It just quietly stops working.

What you actually need to catch this

The standard approach is reactive: a customer complains, you investigate, you find the bug, you fix it, you move on. The problem is that only 10% of users report anything, so by the time you hear about it, the error has likely been running for days or weeks.

What you need is something that captures errors the moment they happen in real users' browsers, groups them into trackable issues, and tells you how many sessions were affected before anyone complained. Then you need it to tell you which errors are worth fixing first, because not every JavaScript warning carries the same weight. An error that hit 3 sessions last week is a different priority to one that's hitting 400 sessions a day in the add-to-cart flow.

This is what Shopify JavaScript error monitoring built specifically for the platform does differently from generic tools like Sentry. It understands Shopify's checkout funnel, tracks errors against the stages where they occur (add-to-cart, checkout initiation, payment submit), and can attribute a revenue impact to each issue based on affected sessions, your conversion rate, and your AOV. That prioritisation is the difference between a developer spending three hours fixing a console warning and spending three hours fixing the thing that's actually costing you £4,000 a week.

The errors most likely to be hurting you right now

Based on the patterns documented across broken Shopify checkouts, here are the most common JS failure types to look for:

Uncaught TypeError: Cannot read properties of null, a script tries to interact with a DOM element that doesn't exist yet, or was removed by another script. Common in theme-app conflicts.
Unhandled promise rejection: Failed to fetch, a network request fails and nothing catches it. Can block cart or checkout UI from updating.
ReferenceError: [variable] is not defined, one script depends on a global set by another, loading order breaks, and the dependency is missing.
Script load failure (net::ERR_BLOCKED_BY_CLIENT), an ad blocker or browser extension blocks a script that your checkout flow depends on. More common than most teams expect.
Checkout pixel silently not firing, post-August 2025 deprecation fallout, no console error, just missing data downstream.
CLS spike from late-loading widget, a third-party script injects an element after paint, shifting the page layout and potentially moving the checkout button just as someone clicks it.

How to start diagnosing your store today

You don't need to wait for a customer complaint. Open your browser's DevTools console on your own storefront and work through the full purchase flow: product page, add-to-cart, checkout, payment, confirmation. Note every error and warning. Then do the same on a mobile device using remote debugging.

What you'll almost certainly find is errors you didn't know existed. The question after that is which ones matter, and that's where manual debugging breaks down. You can reproduce errors in your own session, but you can't know whether the TypeError you just saw affects 2% of sessions or 40% of sessions, or whether it correlates with checkout abandonment.

That's the visibility gap. You can see that errors exist, but you can't see their scale, their distribution across browsers and devices, or their business impact without instrumenting real user sessions.

The monitoring layer your checkout is missing

For stores running significant traffic, especially those with high AOVs or recent app stack changes, operating without real-user JavaScript monitoring is a genuine revenue risk. The 94% figure from Noibu isn't a scare statistic. It means that if your store is in the majority, there are broken things in production right now that you haven't seen yet.

The August 2025 deprecations have added another layer of potential silent failure on top of the pre-existing third-party script chaos. Stores that completed the checkout extensibility migration may still have edge cases. Stores that didn't are running on borrowed time with pixels and customisations that have already stopped working for some users.

Finding these issues requires watching what actually happens in real browsers, across real devices, across the full checkout funnel. Not just checking that your store loads in a test environment. Real monitoring, with error grouping, session counts, and enough context to tell you what broke, where, and what it's costing you before another week of silent abandonment goes by.

Sources

Noibu — "Over 94% of ecommerce websites have 5 or more unresolved JS errors live right now."
Shopify (shopify.com) — "Nearly 72% of online shoppers ditch their carts—with up to 18% doing so simply because checkout is slow or confusing, and another 15% leaving when a page crashes."
Revize — "What's broken right now (April 2026): Pixels and analytics tracking on Thank You and Order Status pages, any Shopify Scripts not migrated to Functions, and all third-party checkout customizations that relied on direct Liquid access."
Invensis — "Conflicting JavaScript functions from upsell widgets, live chat popups, and currency converters often override one another or interfere with theme behavior."
Shopify Dev (shopify.dev) — "Add an unhandledrejection listener for promise rejections or an error listener for other exceptions like Javascript runtime errors or failures to load a resource."
Digital Position — "Shopify is phasing these out to improve checkout speed, reduce the risk of broken or insecure code, and maintain a consistent experience across stores."