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window.onerror does not report unhandled Promise rejections. It handles script errors; a rejected Promise without a rejection handler uses a separate browser event, unhandledrejection. Listen for that event to capture the rejection reason and the Promise that rejected.
Why window.onerror does not catch a rejected Promise
The browser has separate reporting paths for script errors and unhandled Promise rejections. The Window error event reports synchronous script errors. When a Promise rejection has no rejection handler, the browser fires unhandledrejection on the relevant global scope, usually window. MDN notes that this also covers an uncaught throw inside an async function when no rejection handler is attached. See MDN’s Window: error event documentation.
Listen for unhandledrejection
Add a listener to the page’s global object:
window.addEventListener("unhandledrejection", (event) => {
console.error("Unhandled promise rejection:", event.reason);
});The event is a PromiseRejectionEvent. Its reason property contains the rejection value, and promise identifies the rejected Promise. A rejection reason can be any value—not necessarily an Error—so avoid assuming it has properties such as message or stack. The same event can be handled with window.onunhandledrejection, but assigning that property replaces any handler already assigned there. addEventListener allows listeners to coexist. See MDN’s Window: unhandledrejection event documentation.
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It is a fallback for rejections without a handler
Use the global listener as a diagnostic safety net for rejected Promises that escaped local handling. It does not replace handling expected failures where they occur: use try/catch around await when recovery is possible, or .catch() for Promise chains.
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Cross-origin rejections may not reach it
A rejection from a cross-origin script may not dispatch unhandledrejection, because exposing the event could reveal information. A global listener therefore cannot guarantee capture of every rejection.
A rejection may be handled later
If a rejection is reported as unhandled and code attaches a rejection handler afterward, the browser can fire rejectionhandled. Instrumentation can use this event to qualify or update an earlier report instead of treating it as proof that the rejection stayed unhandled. See MDN’s Window: rejectionhandled event documentation.
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Decide whether to suppress the browser’s default report
The unhandledrejection event is cancelable. Calling event.preventDefault() suppresses default handling, such as reporting the rejection in the console. Do so only if your application has deliberately taken responsibility for handling or reporting the failure; otherwise, leave the default behavior intact.
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Use the browser event, not Node.js’s process event
Browser pages use the lowercase event name unhandledrejection. Node.js has a differently capitalized process event, unhandledRejection; its process-listener code is not the browser-page API. For the browser’s event details and handling options, consult MDN’s Window: unhandledrejection event documentation.
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