Debug: Node.js unhandled promise rejections causing crashes

Node.js process crashes with UnhandledPromiseRejection or silently swallows promise errors.

Find and fix unhandled rejections:

// 1. Global handler to catch and log
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
  // In production: log to error tracking, then exit
  process.exit(1);
});

// 2. Common patterns that cause unhandled rejections:

// Missing .catch()
async function fetchUser() { /* might throw */ }
fetchUser(); // No catch! Add: fetchUser().catch(console.error);

// Missing await in try/catch
async function handler() {
  try {
    doSyncThing();
    fetchData(); // Not awaited! Error escapes try/catch
  } catch (e) {
    // Won't catch fetchData errors
  }
}
// Fix: await fetchData();

// Promise.all without catch
Promise.all([fetchA(), fetchB()]); // Add .catch()

// Event emitter async handlers
emitter.on('data', async (data) => {
  await process(data); // Rejection not caught!
});
// Fix: wrap in try/catch
emitter.on('data', async (data) => {
  try {
    await process(data);
  } catch (e) {
    console.error('Processing error:', e);
  }
});

1. Node.js 15+: Unhandled rejections crash by default (--unhandled-rejections=throw)
2. ESLint: Use eslint-plugin-promise with no-floating-promises rule
3. TypeScript: Enable @typescript-eslint/no-floating-promises

Unhandled promise rejections represent bugs where errors are silently lost. Node.js now treats them as crashes by default.

Node.js applications with async/promise-based code