Dev
Mock Error Message Generator
Error states get the least testing effort, yet they are where user trust breaks down. Shipping without validated error handling means broken stack traces in production UIs and toast messages that overflow. A mock error message generator gives you realistic stack traces on demand to test error handling before real failures occur. The `language` input controls the runtime format: JavaScript uses 'at function (file:line:col)' frames, Python uses 'Traceback (most recent call last)' structure, Java shows package-qualified class names, Go produces goroutine panic output, and Ruby uses 'file:line:in method'. The `errorType` input selects the error class — TypeError, NullPointerException, NetworkError, DatabaseError, or AuthError — with language-appropriate messages. A JavaScript AuthError becomes 'JsonWebTokenError: invalid signature'; a Go NetworkError becomes 'dial tcp: connection refused'.
How to use
- Choose your options above
- Click Generate
- Copy your result
Detailed instructions
- Select your target language from the Language dropdown — choose the runtime your error display component is built to handle.
- Pick an error type from the Error Type dropdown that matches the failure scenario you want to test or document.
- Click Generate to produce a formatted stack trace that mirrors real runtime output for that language and error combination.
- Copy the output and paste it directly into your test fixture, Storybook story, mock API response, or documentation screenshot.
Use Cases
- •Populating error states in a UI mockup
- •Testing how toasts and alerts display failures
- •Filling form validation messages during design
- •Illustrating error handling in documentation
- •Stress-testing error layouts with long messages
Tips
- →For Storybook, generate three traces of different lengths — short, medium, and deep — to test text overflow and collapse behavior.
- →Match the language to your frontend or backend runtime; a Java trace in a Node.js error dashboard will look off immediately.
- →When seeding a demo Sentry project, generate five or six different error types and vary the timestamps manually to simulate real traffic patterns.
- →Python tracebacks are multi-line and verbose by default — test that your UI handles wrapping correctly rather than assuming single-line errors.
- →For runbook documentation, generate the specific error type your on-call engineers are most likely to see, not a generic example.
- →If your log pipeline uses regex to parse error levels or exception class names, run the generated output through your parser as a smoke test.
FAQ
how do I generate a realistic stack trace for a specific language and error type
Select your runtime from the Language dropdown and your error class from the Error Type dropdown, then click Generate. The output follows each language's actual stack trace format — JavaScript uses 'at function (file:line:col)', Python uses 'File, line, in function' frames, Java uses package-qualified class names, Go shows goroutine panic output, and Ruby uses 'from file:line:in method'. The error message within each trace is specific to that language-and-error-type combination.
is the generated stack trace format accurate enough to test a log parser or regex
Yes. Each format mirrors the real runtime output closely enough that a log-parsing regex or Grok pattern written against generated traces will work against real ones. JavaScript, Python, Java, Go, and Ruby each have distinct conventions for frame syntax, and the generator follows those conventions. Run several generations to get varied line numbers and function names before finalizing your pattern.
can I use this to test how my ui renders multi-line error messages
Yes — that is one of the primary use cases. Stack traces are multi-line and vary in depth (the generator produces 2–6 frames per trace), so pasting output into a toast component, an error boundary, or a Storybook story immediately reveals layout and overflow behavior that single-line placeholders would never expose. Generate several traces of different lengths to confirm your layout handles both shallow and deep call stacks.
does the generator produce only one error at a time
Yes — the generator outputs a single formatted stack trace per run. There is no count input. To test multiple errors in a Storybook story or a fixture file, click Generate multiple times and copy each output separately.
You might also like
Popular tools from other categories that share themes with this one.
Try these next
More free tools from other corners of the catalog, picked by shared themes.