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When a JavaScript error likeUncaught TypeError: Cannot read properties of undefined (reading 'opacity')
appears on your website, it can be confusing. The good news is Chrome DevTools gives you everything you need to trace the problem directly to its source.
This article explains a practical step-by-step method, based on real debugging of an Elementor issue, to locate, inspect, and fix JavaScript errors using Chrome DevTools.
Key Takeaway
Chrome DevTools is not just a diagnostic tool; it’s a microscope for your JavaScript.
By setting breakpoints and inspecting variables, you can isolate the exact condition causing the problem instead of guessing.
Once you master this workflow, you’ll troubleshoot frontend issues faster, with less frustration, and without needing to modify core plugin files.
1. Open Chrome DevTools
- Visit the page where the error occurs.
- Press F12 or Ctrl + Shift + I (Windows) / Cmd + Option + I (Mac) to open DevTools.
- Go to the Console tab.
You’ll see red text showing the error message, filename, and line number, for example:frontend.min.js?ver=3.23.1:2 Uncaught TypeError: Cannot read properties of undefined (reading 'opacity')
2. Locate the Exact Source
Click the file name (frontend.min.js) next to the error message.
Chrome will take you to the Sources panel and highlight the code that caused the error.
If the file is minified, click the { } Pretty Print button (bottom-left corner). This reformats the code for easier reading.
3. Set a Breakpoint
Set a breakpoint just before the line that throws the error:
- Click the line number where the error appears.
- Reload the page (F5).
- The script will pause automatically at your breakpoint when it executes.
From here, you can inspect variables, DOM elements, and function calls.
4. Inspect the Variables and Objects
While paused:
- Hover over variable names to see their values.
- Look at the Scope panel to check what objects exist.
- If you see a variable that is
undefinedornull, that’s your missing element or object.
In the real example, the code attempted to apply .opacity to a nonexistent background layer in Elementor’s “Scrolling Effect” feature. The background container was missing because no image was set.
5. Step Through the Code
Use the control buttons at the top of DevTools:
- Step over (F10) – run the next line.
- Step into (F11) – dive into the function.
- Step out (Shift + F11) – exit the current function.
This helps you trace how the error occurs in sequence and which function triggers it.
6. Identify the Real Cause
After confirming which object or variable is undefined, return to your website settings or code and fix the source issue.
In this case study:
- The error originated from an Elementor container with Scrolling Effects enabled.
- No background image was set.
- The script still tried to adjust
opacityon a non-existent layer.
Solution: disable the Scrolling Effect or add a background image to the container.
7. Test Again
Reload the page after making the fix.
If the error no longer appears in the Console, clear caches and test in incognito mode to confirm it’s resolved.
8. Bonus Tips for Debugging
- Always test with caching and optimization plugins disabled (WP Rocket, Autoptimize, Cloudflare, etc.).
- Use the Network panel to check if any scripts fail to load.
- Reproduce the issue in Incognito mode to rule out browser extensions.
- Keep Elementor and Elementor Pro versions matched and up to date.
Summary
This article explains how to troubleshoot JavaScript errors using Chrome DevTools, focusing on a real-world Elementor case. It walks readers through identifying and fixing errors such as Uncaught TypeError: Cannot read properties of undefined, using tools like the Console, Sources panel, and breakpoints.
The guide covers how to inspect variables, step through code execution, and trace the source of undefined objects, revealing issues like missing background elements or misconfigured effects. It also provides practical debugging tips for WordPress users, such as disabling optimization plugins and checking script load order.
Ultimately, readers learn a reliable, hands-on workflow for diagnosing frontend JavaScript problems quickly and confidently without editing core files.
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