Start with a repeatable problem
To follow along, you need a desktop browser with built-in developer tools and a page where you can inspect JavaScript. No library or particular framework version is required. Open the browser’s developer tools and select the Console tab. Before changing code, note what you did just before the error appeared: Did you load the page, click a button, submit a form, or navigate somewhere?
Reload the page and repeat those steps. A repeatable sequence helps you tell a persistent bug from an unrelated message. If several messages appear, look for the first error associated with your action rather than assuming the last message caused everything above it. Console output may also include warnings, messages from browser extensions, and logs your own code printed. Those are useful clues, but they are not all JavaScript errors in your page.
Read the message and find the failing line
An error message usually describes what JavaScript was trying to do when it failed. For example, “Cannot read properties of null” suggests that code expected a value but received null instead. The message tells you what failed; it does not necessarily tell you why the value was null.
Next, find the file name and line number linked beside the message. Open that location and inspect the expression on the failing line. If the console shows a stack trace, read it as a trail of function calls leading to the error. Your own function in that trail may be a better place to start than a line inside a library. If the page uses bundled or transformed JavaScript, the displayed file can be difficult to read; source maps, when available, can help developer tools show the original source.
Inspect values instead of guessing
Suppose a page runs this JavaScript: const button = document.querySelector("#save"); button.addEventListener("click", save); If the console reports an error on the second statement, inspect button. When it is null, the problem is not with addEventListener itself: the selector did not find an element at the time the query ran.
Check the likely causes one at a time. Does the element’s id actually match save? Is the element present on this page? Does the script run before the browser has created that element? If timing is the issue, running the script after the relevant element has been parsed is one possible fix. If the element is optional, check that it exists before attaching a listener. Choose a fix that matches the reason it was missing; a null check alone can hide a mistaken selector.
A temporary console.log can show a value just before the failing line. For a closer look, set a breakpoint in the Sources or Debugger panel and repeat the action. When execution pauses, inspect variables and step through statements to see where an unexpected value first appears. Remember that values typed directly into the console may run at a different time from your original script, so they do not always reproduce a timing problem.
Follow the error back to its cause
The line that throws an error is not always the line that introduced the bad value. If a function receives undefined instead of an expected object, look at its caller and the data passed into it. For errors during an asynchronous operation, such as an unhandled promise rejection, inspect the operation and its error-handling path as well as the line named in the message.
Change one thing at a time. A small, focused change makes it easier to tell whether your explanation was right. Avoid clearing the console and declaring success solely because the message disappeared: the code might no longer be running. If the error involves a request or a missing file, the Network panel can offer a separate clue about whether the browser received the resource your code expected.
Verify the fix and leave useful notes
After making a change, reload the page and repeat the original steps. Check both that the error is gone and that the intended behavior works. Then try a nearby case: for a click handler, for example, confirm that it responds to a click and does not fail on pages where its target is absent, if those pages also load the script.
When documenting the bug, record the steps to reproduce it, the relevant error message, what value you found, the cause, and how you checked the fix. Share only the necessary console output; logs can contain private data such as account details or tokens. This short record is useful later because it explains not just what changed, but why.
