Unhandled Promise Rejection

An Unhandled Promise Rejection occurs when a Promise is rejected and there is no .catch() handler or try/catch around an await. In Node.js, this can terminate the process. In browsers, it triggers the unhandledrejection event.

Common causes

• Forgetting to add .catch() to a Promise chain
• Missing try/catch around await in async functions
• Errors inside .then() callbacks without a .catch()
• Rejecting a Promise that nobody is listening to
• Race conditions where error handler is added too late

How to fix it

• Always add .catch() to the end of Promise chains
• Wrap await calls in try/catch blocks
• Add a global unhandledrejection event listener as a safety net
• Use Promise.allSettled() instead of Promise.all() for fault tolerance
• Return Promises from functions so callers can handle errors

Example

// Error example
fetch("/api/data").then(r => r.json());
// If the fetch fails, the rejection is unhandled

async function loadData() {
  const data = await fetch("/api/data");  // Unhandled if fetch rejects
}

// Fix with .catch()
fetch("/api/data")
  .then(r => r.json())
  .catch(err => console.error("Fetch failed:", err));

// Fix with try/catch in async function
async function loadData() {
  try {
    const res = await fetch("/api/data");
    return await res.json();
  } catch (err) {
    console.error("Failed to load:", err);
  }
}

Related errors