JavaScript errors

What we check

We load the home page in a real browser and record what the scripts report:

• Script errors (uncaught exceptions) that stop a script: warning if they come from the website's own domain.
• Error messages (console.error) that scripts print but keep running after: note.
• Errors from third-party scripts: note, listed separately.

Messages about missing files and unencrypted content do not count here, there are separate checks for them.

Why it matters

JavaScript drives almost everything that moves or reacts on a page today: navigation on the phone, image galleries, form validation, cookie notices, maps. If a script stops with an error, everything that comes after it in the same file stops too. Visitors get no message, the page just feels stubborn.

Such errors often creep in: one plugin is updated, another is not; a theme expects a jQuery version that is no longer loaded; an embedded service changes its interface. Because the home page stays visually intact, nobody notices for weeks.

Script errors are also a sign of missing maintenance. The affected files are usually outdated in a security-relevant way as well.

How to fix it

Read the errors

The detail in the report names the first messages. Typical patterns:

• … is not defined or … is not a function: a script relies on another one that is missing or loads too late. Often a missing library such as jQuery or a wrong loading order.
• Cannot read properties of null: the script looks for an element that does not exist on this page. Usually a script meant for another page that is loaded everywhere.
• Unexpected token: faulty code, or a file that is not JavaScript at all, such as an error page instead of the script.

Fix the causes

• Update the CMS and plugins, all of them together. Deactivate plugins that are no longer needed.
• Test after the update: open the home page and the most important subpages with the console open.
• Switch third-party embeds to the provider's current code.

Finding the cause usually needs the person or agency that maintains the website.

How to check it yourself

Open the page, press F12 and switch to the "Console" tab. Reload the page. Red entries are errors; a click on the file name on the right shows which script is affected. Then test whether menu, forms and sliders work.

Frequently asked questions

The page looks fine. Why is a script error a problem?

Because the look comes from HTML and CSS, but the function often from JavaScript. A script that stopped leaves no visible trace, only a menu that does not open, a slider that stands still, or a form that submits nothing.

What is the difference between a script error and an error message in the console?

A script error (exception) stops the script, nothing after it runs any more. An error message in the console is written by a script on purpose, which keeps running. The first weighs more.

Errors from third parties: is there anything to do?

Usually not right away. Check, though, whether the embed is still current. A service that throws errors has often been changed or switched off.