The Silent Conversion Killer
Here's an uncomfortable truth: your website almost certainly has JavaScript errors right now. A study by Raygun analyzed over 1 billion error events and found that the average website experiences JavaScript errors on 3-5% of all page loads. For a site with 100,000 monthly visitors, that's 3,000-5,000 broken experiences every month.
Most of these errors never generate a support ticket. Users don't report "your JavaScript threw a TypeError". They just leave. They click a button that doesn't respond, see a form that won't submit, or encounter a page that looks broken, and they quietly go to a competitor.
Here are the seven JavaScript errors we see most frequently in real-world error tracking data, and each one has a direct impact on conversions.
1. Uncaught TypeError: Cannot Read Properties of Undefined
This is the single most common JavaScript error on the web. It occurs when your code tries to access a property on a variable that is undefined or null.
How it kills conversions: It often happens when an API response doesn't return the expected data structure, or when a DOM element your script expects isn't present on the page (common with A/B test variations or CMS-managed content). The script crashes, and any functionality that depends on it, form validation, add-to-cart buttons, dynamic pricing, stops working.
How to fix it: Use optional chaining (user?.address?.city), add null checks before accessing nested properties, and validate API response shapes before using them. Implement defensive coding practices for any data that comes from external sources.
2. Cross-Origin Resource Sharing (CORS) Errors
CORS errors occur when your frontend code tries to make a request to a different domain and the server doesn't include the proper headers to allow it.
How it kills conversions: CORS errors can silently break payment processing, form submissions to third-party services, analytics tracking, and any API call to a different domain. The user clicks "Pay Now" and nothing happens. The form submits but the data never reaches your CRM.
How to fix it: Ensure your API servers return the correct Access-Control-Allow-Origin headers. If you're calling third-party APIs from the browser, use a server-side proxy. Test cross-origin requests in all environments, CORS issues often only appear in production when the domain is different from your development setup.
3. Script Loading Failures
External scripts, payment processors, chat widgets, analytics tools, social media embeds, can fail to load due to network issues, ad blockers, content security policies, or CDN outages.
How it kills conversions: If your checkout depends on Stripe.js loading and it fails, the entire payment flow breaks. If your form validation library doesn't load, forms either don't validate (submitting garbage data) or don't submit at all. According to HTTP Archive data, the median web page loads 21 external JavaScript resources, each one is a potential point of failure.
How to fix it: Never assume external scripts will load. Add error handlers for script loading, implement fallbacks for critical functionality, and test your site with scripts blocked. For payment processing, always implement server-side validation as a backstop.
4. Unhandled Promise Rejections
As more JavaScript moves to async/await and Promises, unhandled rejections have become one of the fastest-growing error categories. A Promise rejects (usually due to a failed API call or timeout), and no .catch() or try/catch handles it.
How it kills conversions: Unhandled rejections crash async workflows, checkout flows, search-as-you-type, dynamic content loading, and form submissions. The user action triggers the Promise, it fails silently, and the UI never updates. No loading spinner disappears, no success message appears, no error message explains what went wrong.
How to fix it: Add .catch() to every Promise chain and wrap every await in a try/catch. Add a global unhandledrejection event listener to catch any that slip through. Always provide user-facing error states for async operations.
5. Event Listener Errors on Dynamic Content
When you attach event listeners to elements that are dynamically added to the page (via AJAX, SPA routing, or framework rendering), timing issues can cause listeners to not be attached, or to be attached to the wrong element.
How it kills conversions: Buttons that don't respond to clicks, forms that don't submit, modals that don't open. This is especially common in single-page applications where content is rendered after the initial page load. A user navigates to the checkout page via client-side routing, and the "Place Order" button has no click handler attached because the event listener was only set up on initial page load.
How to fix it: Use event delegation (attach listeners to parent elements that persist in the DOM). In frameworks like React, Vue, or Angular, ensure event handlers are properly bound in component lifecycle hooks. Test your entire flow via client-side navigation, not just direct page loads.
6. Local Storage / Cookie Quota Errors
When localStorage or cookies exceed their storage quota, subsequent write operations throw errors. The typical localStorage limit is 5-10MB per origin, and cookies are limited to about 4KB each with a per-domain cap.
How it kills conversions: Shopping carts stored in localStorage can fail to update. Session tokens can fail to persist, causing random logouts. Form progress saved locally can disappear. Users on mobile devices with limited storage are disproportionately affected. The error rarely produces a visible failure, the cart just silently loses items or the login state randomly resets.
How to fix it: Always wrap localStorage operations in try/catch blocks. Implement a storage management strategy, clean up old data, set expiration for cached items, and handle quota errors gracefully with user-facing feedback.
7. Race Conditions in Asynchronous Code
When multiple asynchronous operations compete for the same resource or depend on each other's results without proper coordination, you get race conditions. The code works most of the time but fails intermittently.
How it kills conversions: These are the most insidious errors because they're intermittent. A double-click on the "Place Order" button submits two orders. A search request returns results for the previous query. A form submits before validation completes. Because they only happen sometimes, they're hard to reproduce and easy to dismiss.
How to fix it: Disable buttons after click until the operation completes. Cancel previous requests when a new one is made (using AbortController). Use mutex patterns or debouncing for operations that shouldn't run concurrently. Test with network throttling to make race conditions more likely to surface.
How to Catch These Before Your Users Do
The fundamental problem isn't that these errors exist, they're somewhat inevitable in complex web applications. The problem is that most teams don't know about them until user complaints trickle in.
Effective error tracking requires three things:
- Automatic capture: Every unhandled error and rejection should be logged automatically, with full stack traces and browser context.
- User session context: An error log is useful. An error log linked to a session replay showing exactly what the user was doing when the error occurred is transformative. Spectry connects error events directly to session recordings, so you can see the user's full experience. Not just the stack trace.
- Impact measurement: Not all errors are equal. An error that occurs on your blog sidebar is less urgent than one that breaks your checkout form. Prioritize by page importance, user volume, and conversion impact.
Run an error audit this week. Check your error tracking for these seven error types, prioritize by conversion impact, and fix the top three. You'll likely recover conversions you didn't even know you were losing.