Console errors on your landing page: which ones break signups

The console errors that break signups are the ones thrown on the path from your landing page to a submitted form: an uncaught exception while the page loads or inside a click handler, an unhandled promise rejection on a data fetch, and a failed script that your button depends on. Deprecation warnings, messages from browser extensions and a missing favicon are noise you can leave for later.

You find the dangerous ones by walking the conversion path in a clean browser profile with an ad blocker switched on, and you keep finding them by listening for both the error and the unhandledrejection events in production. The rest of this guide sorts the errors you will see and shows the test and the logging code.

Which console errors break a signup?

An error breaks a signup when it stops code that the signup needs from running: the script that attaches the button's click handler, the fetch that loads the plan list, or the library that submits the form. The same message can be harmless on a blog post and fatal on your pricing page, so judge each error by where it fires.

The KeyCDN support article on ERR_BLOCKED_BY_CLIENT explains why that row surprises people: ad-blocking filter lists match text in the URL, so a first-party file whose path contains a word such as ads can be blocked even though it is yours.

Why can one uncaught error kill a form or CTA?

Because an exception stops the code after it. MDN's description of throw is exact: execution of the current function stops, control passes to the first catch in the call stack, and if there is none the program terminates. Any line after the throw that would have attached a listener, filled a select box or enabled a button never runs.

That is why these failures are silent. The page has already painted, the button is visible and styled, and nothing tells the visitor that its handler was never attached. Two patterns produce this again and again:

1. A setup script that does several jobs in order, such as reading a query parameter, starting analytics and then binding the signup form. If the analytics call throws, the form binding below it is skipped.
2. A click handler that calls something before it submits, such as analytics.track('signup_click') followed by form.submit(). If analytics is undefined because its script was blocked, the handler throws on its first line and the form never submits.

In a React app the unit of failure is bigger. React's documentation says that by default, if your application throws an error during rendering, React removes its UI from the screen. An error boundary limits the damage to the subtree it wraps, but everything under it is still replaced by its fallback, so a single bad value in a pricing card can take the whole pricing section with it.

Why don't unhandled promise rejections reach window.onerror?

They fire a different event. MDN states that the window error event is only generated for errors thrown synchronously, during initial loading or inside event handlers. A Promise that rejects with no handler, including an uncaught throw inside an async function, fires unhandledrejection on window instead.

This matters because most async failures on a landing page are fetches: the plan list, the geolocated currency, the "spots left" counter, the signup POST itself. When one rejects without a handler, the component waiting on it often stays on its loading skeleton forever and nothing reaches an onerror logger. The browser still prints Uncaught (in promise) to the console, so you see it in DevTools and your monitoring does not.

Listen for both events:

function report(kind, detail) {
  const body = JSON.stringify({ kind, detail, page: location.pathname })
  navigator.sendBeacon('/api/client-errors', body)
}

window.addEventListener('error', (e) => {
  report('error', `${e.message} at ${e.filename}:${e.lineno}:${e.colno}`)
})

window.addEventListener('unhandledrejection', (e) => {
  report('unhandledrejection', String(e.reason))
})

navigator.sendBeacon is built for this kind of small report: it queues a POST without delaying navigation, and the queued data is limited to 64 KiB. A failed image or script load fires error on the element itself, and MDN notes that this event does not bubble, so a plain window listener misses most of them; check the Network panel for those. Google's JavaScript troubleshooting guide recommends this kind of collection too, noting that the errors Googlebot hits on your site are among the ones worth logging.

How do you stop a third-party script from disabling your button?

Make every first-party control work without the vendor. Chat widgets, analytics, A/B testing tools and schedulers can fail to load because of an ad blocker, a content security policy, an outage or a slow network, and your signup button should not care. Four changes cover almost every case.

1. Use a real destination. A "Book a demo" control should be an <a href="/demo"> that navigates without JavaScript, with the scheduler widget added on top when it loads.
2. Guard every vendor call. Write window.Intercom?.('show') or check typeof window.Intercom === 'function' before calling, and fall back to a mailto: link or your contact page.
3. Wrap vendor initialisation in try/catch, so an exception inside their setup cannot stop yours.
4. Load vendor scripts with async or defer. MDN describes async as fetching in parallel and running as soon as the script is available, and defer as running after the document is parsed, in order. Neither blocks the parser from reaching your own markup.

Track events after the action, never before it. Submit the form first and send the analytics event from the success path, or send it with sendBeacon, so a missing analytics library cannot stand between a visitor and your signup.

How do you test the conversion path for console errors?

Walk the path a new visitor takes, in a browser that looks like theirs, with the console set to show only errors. Your everyday profile hides problems: it has cached files, a logged-in session and whatever extensions you use. Run this before a launch and after any release that touches the homepage, pricing or signup.

1. Create a fresh Chrome profile, or open a Guest window, so nothing is cached or signed in.
2. Install one mainstream ad blocker in it and leave it on, so you load the page the way visitors with blocked scripts do.
3. Open DevTools, go to Console settings and tick Preserve log, so messages survive each navigation. Set the Log Levels drop-down to Errors only.
4. Load the homepage, click the primary CTA, visit pricing, pick a plan, and submit the signup form with test data.
5. Record every error with the page it appeared on. In the Console sidebar, User Messages shows only what came from the page's own JavaScript.
6. Open the Network panel, right-click each third-party request and choose Block request. Repeat step 4 with the chat widget, analytics and scheduler blocked one at a time. Blocking stays active only while DevTools is open.
7. Repeat the walk on your phone, or in DevTools device mode, because mobile menus and sticky bars run code that the desktop layout never executes.

Anything in the "Yes" rows of the table above that you found on this walk is a signup bug. Fix it before the rest. Then click each primary control with the keyboard as well as the mouse, using the pre-launch test for dead CTA buttons.

Which console errors can you safely ignore?

Errors that come from outside your code, and warnings about code that still works, can wait. Ignore messages whose source is a browser extension, [Violation] messages about long handlers (useful for speed work, not for signups), deprecation notices for APIs that still run, and a 404 for favicon.ico or a source map.

Do not ignore something only because it appears on every page. An error in a shared layout or a global script runs on the signup page too. And do not treat Script error. as noise: MDN explains that the browser passes minimal information to window.onerror for scripts that do not pass CORS checks, so the message is hiding a real error. Add the crossorigin attribute to that script tag, serve the file with an Access-Control-Allow-Origin header, and the full message appears.

A 404 in the console is worth one more look. If the missing URL is a page rather than a file, some link on your site points at it, and visitors and crawlers are following that link to a dead end; the guide to finding broken internal links covers the sweep.

Find the errors on your live conversion path

A walk in DevTools shows the errors your own browser hits on the day you test. To have your live pages checked the way a first-time visitor's browser loads them, run the free scan first, then open the full audit. The free LaunchScaler scan takes only your URL, needs no account, and runs 156 checks across 6 of its 7 categories. Its does-it-work category loads the page in a real browser and includes "Console errors accumulate along the conversion path", "Unhandled promise rejection on a data-fetch path" and "Third-party script failure degrades a first-party feature", beside checks for dead CTAs and forms with no way to submit.

The free run gives each check its verdict. The full audit, $19 one time for your domain, opens every check to its evidence and its exact fix: which errors fired, on which step of the path, and what to change so the signup survives them.