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First identify which layer is failing
A browser or API client may show an error even when the underlying problem is not an application-generated HTTP 500. The failure may occur during installation or build, while the Node.js process starts, inside a route or other request handler, or at the host’s router or reverse proxy. These cases need different fixes.
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- Record the affected route, status code, time, deployment revision, and whether the failure happens on every request or only some requests.
- Compare the build or deploy output with runtime logs from the same time. Find the first relevant error or exception rather than relying only on the final status message.
- Check whether the Node.js process exited. If it did, note its exit code and the last runtime messages. If it remains up, check whether one route or all routes fail.
- If the app process appears healthy but requests fail, inspect the provider’s router or proxy logs for upstream or connection errors.
For example, Heroku documents an H10 / “App crashed” router entry that returns HTTP 503 when an app repeatedly crashes. That is a Heroku-specific symptom, not a universal code for 500 errors or a definition of what another provider will report. See Heroku’s H10 error documentation.
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Check production startup and deployment configuration
When an app builds but does not stay up, verify that production launches the intended entry point and that its runtime requirements exist in the deployed environment.
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- Start command: Confirm that the host’s configured command runs the intended server entry point, not a development script or a build-only command.
- Production dependencies: Make sure packages imported at runtime are installed as production dependencies. Heroku prunes
devDependenciesfrom the deployment slug, so a module placed there may be available locally but missing after deployment. Heroku recommends reproducing install or build issues in an environment based on the deployed slug. See Heroku’s Node.js deployment troubleshooting guide. - Environment configuration: Check that required production values are present under the exact names the app reads. Compare presence or safe metadata without printing secret values into logs. Environment-variable setup differs by host, so follow the current documentation for your provider.
- Port and listener: Verify that the server listens on the port expected by the host. On Heroku, the app should use
process.env.PORT, optionally with a local development fallback; binding to an unsuitable fixed port can cause repeated crashes. This is Heroku-specific guidance, so confirm the equivalent requirement for another host. See Heroku’s web dyno documentation.
Read the first exception or rejection in the runtime logs
By default, Node.js prints an uncaught JavaScript exception and its stack trace to stderr, then exits with code 1. Under documented rejection settings, an unhandled promise rejection can also become the origin of an uncaught exception. Find the first stack frame in your application or a dependency, then inspect the values and production assumptions used there. The Node.js v26.10.0 process documentation describes this default behavior; check the documentation for the release actually deployed if it differs.
Do not use a broad uncaughtException handler to keep serving requests. Node.js warns that it is not safe to resume normal operation after an uncaught exception because the program may be in an undefined state. If necessary, perform synchronous cleanup and shut down; use an external monitor in a separate process to detect failure and restart or recover the app. See the Node.js process documentation.
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Generate a diagnostic report if logs do not explain the crash
When ordinary logs do not reveal why the process failed, Node.js diagnostic reports can capture JavaScript and native stack traces, heap statistics, platform details, and resource usage. They can be triggered for uncaught exceptions, fatal errors, or signals, subject to the deployed Node.js version and platform. The Node.js v26.10.0 documentation describes these options:
--report-uncaught-exceptionwrites a report for an uncaught exception.--report-on-fatalerrorwrites a report for a fatal runtime error, which may help investigate problems such as out-of-memory termination.--report-on-signalenables report generation on a signal. Signal-based generation is not supported on Windows.
See the Node.js diagnostic report documentation and check that the relevant flags are supported by your deployed release and host. Reports include environment variables by default; use --report-exclude-env to omit them. Treat reports as sensitive files and do not publish or share secrets they contain.
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Match the next action to what you observe
| What you observe | Where to investigate |
|---|---|
| The deployment fails during install or build | Build output, package installation, build scripts, and whether the build environment matches the deployed runtime. |
| The process exits before it can serve requests | Runtime startup logs, start command, production dependencies, environment configuration, port binding, and the first exception or fatal error. |
| The process stays up but one route returns 500 | The route’s application logs and stack trace, plus the inputs and production values used by that handler. |
| The process seems healthy but the host reports an upstream failure | Provider router or proxy logs, including their status code and connection details; interpret these using that provider’s documentation. |
| Logs do not show why the process terminates | Node.js diagnostic reports, after checking flag support and protecting any sensitive report contents. |
The cause of a particular incident cannot be determined from the 500 status or the fact that deployment succeeded alone. Use the timestamp, route, process state, and first relevant runtime or router error to narrow the failure to its layer before changing code or host settings.
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