How to Detect JavaScript Errors That Are Costing You Customers

Your website is probably breaking for some users right now. Learn how to detect JavaScript errors that kill conversions before your users silently leave.

Your website is broken for some of your users right now.

Not for you. Not in your tests. Not in Chrome on your MacBook with fast Wi-Fi.

For them. The real ones. The ones paying attention on an older Android phone with a browser extension installed, or on a flaky 3G connection on a Tuesday afternoon.

And the worst part — they're not telling you.

What a JavaScript error actually looks like from a user's side

From your side, the site works fine. You tested it. Your developer tested it. QA passed.

From the user's side, they clicked the signup button and nothing happened.

They didn't get an error message. They didn't see a red warning. The button just didn't work. So they did what every user does when something silently fails — they assumed it was them, felt mildly embarrassed, and left.

Your analytics recorded a drop-off. No reason. No context. Just gone.

That's a JavaScript error. Invisible to you. Invisible to your tools. Completely visible to the person who just decided not to come back.

Why your testing environment lies to you

Every developer tests locally. Every founder checks the site before launching. Most run it through Chrome DevTools, maybe Lighthouse, maybe a quick phone check.

None of that replicates what your users actually experience.

Real users have ad blockers that conflict with your scripts. They have browser extensions that intercept form submissions. They're on Chrome 89 because their company IT department never updated. They're on a VPN that slows asset loading to the point where your JavaScript times out before it initialises.

You can't test for every combination of those variables. But your users are living in them every day.

A JavaScript error in a controlled environment is rare. In the wild, across thousands of different browser and device configurations, they happen constantly — and silently.

The three ways to catch them — and why two of them aren't enough

Method 1 — window.onerror

You can add a listener to your site that captures errors as they happen:

window.addEventListener('error', function(event) {
  console.log('Error caught:', event.message, 'on line:', event.lineno);
});

This tells you what broke and where in your code. Useful for developers. Completely useless for understanding the user impact. You'll see TypeError: Cannot read properties of undefined and have no idea whether that error happened on the checkout page for 3 people or the homepage for 300.

Method 2 — Error monitoring tools (Sentry, LogRocket)

These are the professional tools for tracking JavaScript errors at scale. They give you stack traces, session replays, affected user counts. Sentry alone is a genuinely excellent product.

The problem is cost ($26/mo starting, scaling to hundreds), complexity for non-developers, and a fundamental gap they all share: they tell you what broke technically. They don't tell you what the user was trying to do when it broke. They don't tell you whether the broken thing was critical or irrelevant to the conversion.

Method 3 — Ask the user at the moment it breaks

This is the one nobody does and the one that gives you the most useful information fastest.

When a JavaScript error fires on your site, your feedback widget detects it and immediately asks the user one question: "Did something break on this page?"

A user who just hit a broken button is already frustrated. They have a specific answer. They will tell you "I clicked sign up and nothing happened" or "the payment form wouldn't load" in plain English — which is infinitely more actionable than a stack trace.

This is what a 15 to 25% response rate looks like. Not because the widget is clever. Because the timing is perfect. You're asking the person who just experienced the problem, at the exact moment they experienced it.

Why the combination wins

Error monitoring tools tell you what broke in your code.

User feedback tells you what broke in their experience.

One of those things is a debugging task. The other is a revenue problem.

A founder running a SaaS noticed drop-offs on their signup page for weeks. Error logs showed nothing critical. It was only after adding error-triggered feedback that they got the answer: "Button doesn't work on my phone." A single CSS z-index issue was stacking an invisible div over the signup button on mobile Safari. It had been there for six weeks. Every mobile user who tried to sign up in that window hit a button that looked fine and did nothing.

Fixed in 20 minutes once they knew what to look for.

Found in 48 hours once they started asking.

What to do right now

You don't need to install Sentry. You don't need to hire a developer.

Add Loopnote's widget to your site with one script tag. Turn on the JavaScript error trigger in the dashboard. When an error fires on any page, your users automatically get asked what happened.

You'll know about broken things before they cost you another month of lost signups.

The errors are already there.

You just need to start listening to the people hitting them.