Error Handling Patterns: Beyond try/catch

class AppError extends Error {
  constructor(message, code, statusCode = 500) {
    super(message);
    this.name = "AppError";
    this.code = code;          // machine-readable, e.g. "NOT_FOUND"
    this.statusCode = statusCode;
    Error.captureStackTrace(this, this.constructor);
  }
}

class ValidationError extends AppError {
  constructor(message, fields = {}) {
    super(message, "VALIDATION_ERROR", 400);
    this.name = "ValidationError";
    this.fields = fields;
  }
}

// Usage
async function getUser(id) {
  if (!id) throw new ValidationError("id is required", { id: "missing" });
  const user = await db.find(id);
  if (!user) throw new AppError("User not found", "NOT_FOUND", 404);
  return user;
}

// Catch unhandled Promise rejections (Node.js 15+ terminates by default)
process.on("unhandledRejection", (reason, promise) => {
  console.error("Unhandled rejection at:", promise, "reason:", reason);
  // Log to monitoring (Sentry, Datadog, etc.) then shut down gracefully
  process.exit(1);
});

// Catch synchronous exceptions that escaped all try/catch blocks
process.on("uncaughtException", (err) => {
  console.error("Uncaught exception:", err);
  // Do NOT continue running — the process state is undefined
  process.exit(1);
});

// Graceful shutdown on SIGTERM (sent by Docker, Kubernetes, etc.)
process.on("SIGTERM", async () => {
  console.log("SIGTERM received — shutting down gracefully");
  await server.close();
  process.exit(0);
});

// Return { data, error } instead of throwing — avoids hidden control flow
async function safeReadFile(filePath) {
  try {
    const data = await require("fs/promises").readFile(filePath, "utf8");
    return { data, error: null };
  } catch (err) {
    return { data: null, error: err };
  }
}

const { data, error } = await safeReadFile("config.json");
if (error) {
  console.error("Could not load config:", error.message);
} else {
  console.log("Config loaded:", data);
}