Node.js EADDRINUSE first triage response

When this happens

Use this when only the visible message is known. Do not stop at the screen message; validate the first observable mismatch before any setting is changed first.

Symptom checklist

• EADDRINUSE appears repeatedly in the Node.js UI or logs.
• process env, npm tree, memory, port, module type differs between successful and failed requests.
• The issue appears only after separating Node runtime error from package dependency issue.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• EADDRINUSE specifically changes the investigation surface for Node.js: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• Another process already listens on the configured port.
• A stream emits error or backpressure without an error handler.
• Worker threads or cluster workers exit without propagating the root error.
• The process manager restarts quickly and hides the first stack trace.
• Environment variables differ between shell, service, and container runtime.
• For the first triage case, the first useful clue is the first observable mismatch before any setting is changed.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare process env, npm tree, memory, port, module type for success and failure in the same window.
3. Test the hypothesis: Node.js runtime failure caused by occupied port, stream backpressure, worker lifecycle, unhandled error event, or process manager restart loop.
4. Classify this as first triage: the first observable mismatch before any setting is changed.
5. Capture current values before changing configuration.

First evidence

Treat EADDRINUSE as a first triage case. First collect evidence for the first observable mismatch before any setting is changed.

Output examples

Normal output

No matching error appears during the affected request window.

Failing output

The same error appears at the exact request time with the affected path, user, or object.

Output-to-action branches

• Only the visible message is known.
  Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.
• The working and failing outputs differ.
  Act on the differing layer first: For EADDRINUSE, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not change DNS, SSL, cache, and permissions at the same time.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "EADDRINUSE" ./logs

node --trace-warnings server.js

npm ls

lsof -i :3000

grep -R "EADDRINUSE" src logs

lsof -i :3000 || netstat -ano | findstr :3000

node --trace-warnings server.js

grep -R "EADDRINUSE\|stream\|worker\|unhandled" ./logs src

npm ls

date -u && grep -R "ERROR\|WARN\|failed" ./logs | tail -n 50

Fix order

1. Record the full EADDRINUSE message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for Node.js runtime failure caused by occupied port, stream backpressure, worker lifecycle, unhandled error event, or process manager restart loop.
3. Compare the failing case with a successful case before editing settings.
4. If this is the first triage branch, Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For EADDRINUSE, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Find the listener on the port before changing application code.
• Add error, close, and backpressure handling around streams.
• Log worker exit codes and messages at the parent process.
• Read the first crash in process manager logs.
• Print effective runtime environment during startup.
• For the first triage branch, Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.

Evidence links

• Node.js errors official
• Node.js diagnostics official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for Node.js EADDRINUSE.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change Node runtime error from package dependency issue results.
• Do not trust only the Node.js UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for process env, npm tree, memory, port, module type.
• Add npm dedupe, ESM/CJS alignment, heap tuning, port cleanup to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Node.js ECONNRESET first triage response
• Node.js MODULE_NOT_FOUND first triage response
• Node.js ERR_REQUIRE_ESM first triage response
• Node.js heap out of memory first triage response
• Node.js npm peer dependency first triage response
• Node.js fetch failed first triage response
• Node.js timeout first triage response
• Node.js stream error first triage response