Handling Errors

Iron Laws

1. Never swallow errors – Empty catch blocks hide bugs
2. Never convert errors to booleans – Loses all context
3. Preserve error context when wrapping or propagating
4. Log once where handled, not at every layer

Error Messages

Every error message answers: What happened? Why? How to recover?

For logs (developers):

logger.error("Failed to save user: Connection timeout after 30s", {
  userId: user.id,
  dbHost: config.db.host,
  error: error.stack,
});

For users:

For user-facing error copy, use Skill(ce:writer) with The UX Writer persona. Key principles:

• Brief and specific (not “Something went wrong”)
• Actionable (tell them what to do next)
• No blame (never “You entered invalid…”)

showError({
  title: "Upload failed",
  message: "File exceeds 10MB limit. Choose a smaller file.",
  actions: [{ label: "Choose file", onClick: selectFile }],
});

Error Categories

Fail Fast vs Degrade Gracefully

Fail fast for critical dependencies:

await connectToDatabase(); // Throws on failure - app can't run without it

Degrade gracefully for optional features:

const prefs = await loadPreferences(userId).catch(() => DEFAULT_PREFS);

Log at the Right Layer

// ❌ Logging at every layer = same error 3x
async function fetchData() {
  try { return await fetch(url); }
  catch (e) { console.error("Fetch failed:", e); throw e; }
}

// ✅ Log once where handled
async function fetchData() {
  const response = await fetch(url);
  if (!response.ok) throw new Error(`HTTP ${response.status}`);
  return response;
}
// Top level logs the error once

Language-Specific Patterns

• TypeScript/React: See reference/typescript-react.md for Error Boundaries, typed errors, Result pattern, UI display
• Python: See reference/python.md for EAFP, exception chaining, context managers
• Go: See reference/go.md for explicit error returns, wrapping with %w, sentinel errors

Anti-Patterns