Unhandled rejection handling in Node.js - Performance & Optimization

Performance: Unhandled rejection handling

HIGH IMPACT

This affects the responsiveness and stability of Node.js applications by preventing crashes and unresponsive states caused by unhandled promise rejections.

Handling promise rejections in asynchronous code

Node.js

async function fetchData() {
  try {
    const data = await fetch('https://api.example.com/data');
    return await data.json();
  } catch (error) {
    console.error('Fetch failed:', error);
    return null;
  }
}

fetchData();

Catches errors preventing unhandled rejections and keeps event loop responsive.

📈 Performance GainPrevents event loop blocking and process crashes

Handling promise rejections in asynchronous code

Node.js

async function fetchData() {
  const data = await fetch('https://api.example.com/data');
  return data.json();
}

fetchData(); // No catch or error handling

Unhandled promise rejection can crash the Node.js process or cause unresponsive behavior.

📉 Performance CostBlocks event loop on error, causing delayed response or crash

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No rejection handling	N/A	N/A	N/A	[X] Bad
Local try/catch with async/await	N/A	N/A	N/A	[OK] Good
Global unhandledRejection logging	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Unhandled promise rejections affect the Node.js event loop by potentially blocking it or causing crashes, which delays processing of other events and degrades responsiveness.

→Event Loop

→Error Handling

⚠️ BottleneckEvent Loop blocking due to unhandled errors

Core Web Vital Affected

INP

This affects the responsiveness and stability of Node.js applications by preventing crashes and unresponsive states caused by unhandled promise rejections.

Optimization Tips

1Always handle promise rejections locally with try/catch or .catch()

2Use process.on('unhandledRejection') to log and monitor missed errors

3Never ignore unhandled rejections to avoid event loop blocking or crashes

Performance Quiz - 3 Questions

What is the main performance risk of not handling promise rejections in Node.js?

A The browser will repaint more often

B The event loop can block or crash the process

C CSS selectors become slower

D The network requests increase in size

DevTools: Node.js Inspector (Debugger) and Console

How to check: Run your Node.js app with --inspect flag, open DevTools, and monitor console for unhandled rejection warnings or errors.

What to look for: Look for 'UnhandledPromiseRejectionWarning' or logged errors indicating unhandled rejections.