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Start with the browser console
- Reproduce the action that triggers the problem, such as loading a page, submitting a form, or clicking a control.
- Open the browser’s developer tools and select the Console. The exact menu path and labels vary by browser.
- Find the error that appeared during the reproduction. Record its message and the linked file and line or location.
- Open the source location and inspect the indicated statement along with the values it reads or calls.
A JavaScript error has a name and a message, and a console entry may link to a reference for that message. Treat the wording as a clue, not a complete diagnosis: the underlying issue may be an unexpected value, a missing prerequisite, or code running at the wrong time. See MDN’s JavaScript error reference.
Identify what kind of failure you have
Syntax errors
A syntax error means JavaScript could not parse the code. Look near the reported location for malformed syntax, a missing delimiter, or another invalid construct. A linter can also catch some invalid code before you run it.
Runtime errors
A runtime error occurs while code is executing. The syntax may be valid, but an operation fails—for example, because a value is missing or does not have the shape or type the code expects. Inspect the failing expression and the values it depends on.
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Logic errors
A logic error can produce the wrong result without throwing an exception. If the feature behaves incorrectly but the console shows no relevant error, trace the inputs and decisions that lead to the result rather than waiting for an error message. MDN explains these distinctions in “What went wrong?”.
Inspect values, then step through the code
Log the values around the failure
Add a temporary console.log() or console.error() before the failing operation to see the values the code actually receives. Check that each value is present, has the expected type and shape, and is ready to use. Remove temporary diagnostic logging when it is no longer needed.
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Pay particular attention to asynchronous work. For example, fetch() returns a Promise; it does not immediately return the resolved response data. Code that treats the Promise as if it were already JSON can make incorrect assumptions about the value available at that point.
Use a breakpoint when logs are not enough
Set a breakpoint on or just before the statement that fails, reproduce the issue, and let execution pause. Inspect the current scope to see local values, then follow the call stack to understand how execution reached that statement. MDN’s debugging and error-handling guide covers console inspection, breakpoints, and related techniques.
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Handle asynchronous errors separately
A synchronous uncaught exception and an unhandled Promise rejection are different failure paths. The window error event reports synchronous script errors; an unhandled Promise rejection uses unhandledrejection. A global handler that listens only for error will not necessarily capture rejected Promises. Handle failures where the asynchronous operation is used, and account for unhandled rejections separately if you rely on global error reporting. See MDN’s Window: error event documentation.
When the console reports a CORS error
Inspect the failed request in the Network panel and check the server’s response configuration. Cross-origin access is controlled by browser security rules, and the server must allow the requesting origin. A change to page JavaScript alone usually cannot grant that permission; the fix typically requires changing the server configuration. If you do not control the remote server, a proxy you control may be an option.
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Setting a request to mode: "no-cors" does not make a blocked response readable. It produces an opaque response, whose body and headers are unavailable to JavaScript. Consult MDN’s CORS errors guide before changing request code.
When an error points into minified or bundled code
Optimized JavaScript can make a console location difficult to connect to the source you wrote. Check whether the deployed file has a valid source map and whether the developer tools can load it. A source map lets tools map generated code back to the original source. The SourceMap response header can identify a map; MDN notes that it takes precedence over a source annotation when both are present. See MDN’s SourceMap header reference.
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- Correct the cause indicated by the code and runtime values, not just the visible wording of the message.
- Repeat the same action that originally triggered the error.
- Check that the relevant console error is gone and that the affected feature now behaves as intended.
If the issue persists, reduce the problem to the smallest reproducible case you can, then inspect its inputs, timing, and call path again. A syntax fix will not correct a data assumption; a client-side change will not override a server’s CORS policy; and an error handler will not automatically cover every kind of asynchronous failure.
Or skip the browser setup
For capturing a page screenshot rather than diagnosing its JavaScript, ScreenshotNeo offers a one-call API. For example, this cURL request saves a WebP screenshot of a page:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp
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See the ScreenshotNeo API documentation for the request options. ScreenshotNeo removes cookie banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, and failed loads are never billed. Its MCP server lets AI agents take screenshots, and 1,000 screenshots a month are free with no card; paid plans start at $5 for 3,000. Learn more at ScreenshotNeo, or sign up for the free plan.
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