JavaScript issues blocking vendor site features

Hit a situation where the order checkout button was completely non-functional on a vendor site for two days before I figured out the cause. Documenting this because the fix was counterintuitive.

The symptom: clicking the checkout button did nothing. No loading indicator, no error message, just silence. The same behavior on Firefox and on Chrome incognito. I could browse the site and add items to cart but the checkout process would not advance past the cart page.

What was causing it

The vendor's site used a third-party payment processing widget that loaded JavaScript from an external domain. My ad blocker was blocking that external domain entirely, which silently broke the checkout button — there was no visible error because the block happened before the script could load any error handlers.

The fix was disabling my ad blocker for that specific vendor domain only. After doing that, the checkout page loaded an additional element that had been invisible before, and the button worked immediately.

Common causes of vendor site JS failures

Ad blockers are the most common cause. They use domain blocklists that include many legitimate services used by vendor sites — analytics, payment processors, CDN-hosted libraries. When these are blocked, site functionality breaks unpredictably. The site may load but certain features silently fail.

Browser privacy extensions that block cross-site cookies can also break checkout flows that depend on session state from a third-party payment or captcha service.

Content Security Policy misconfigurations on the vendor's end can cause their own scripts to fail — this appears as console errors if you open browser developer tools, which is a useful first diagnostic step when a site seems broken.

Quick diagnostic steps

Open browser developer tools (F12), go to Console tab, and check for red error messages when clicking the broken element. This usually identifies the blocked or failing resource. Then try temporarily disabling extensions to see if any are causing the issue. Test in a fresh incognito window with extensions disabled as a quick baseline.