What is a UnhandledPromiseRejection? | Lingua-e

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An UnhandledPromiseRejection occurs when a Promise is rejected but no .catch() handler or try/catch inside an async function catches the error. In Node.js it will crash the process by default in recent versions. In browsers it fires the unhandledrejection event.

Why it happens

•Calling an async function without awaiting it or attaching a .catch() handler.
•An awaited call throws inside an async function that has no surrounding try/catch.
•A Promise chain that throws in a .then() callback but the chain has no .catch() at the end.
•Forgetting to return a Promise from a function, leaving a floating promise in a chain.

Examples

JavaScript

Code that triggers the error

async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}

// Called without catch: rejection is unhandled
fetchUser(999);

Error output

UnhandledPromiseRejection: Error: HTTP 404

How to fix it

Always attach a .catch() to every Promise chain, or use try/catch inside async functions. Set up a global handler (process.on('unhandledRejection', ...) in Node.js or window.addEventListener('unhandledrejection', ...) in the browser) as a safety net to log unexpected rejections. Never leave a floating promise that discards its result.

Fixed code

async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}

// Always handle the rejection
fetchUser(999).catch((err) => {
  console.error("Failed to fetch user:", err.message);
});

Related errors

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