WorkerGlobalScope: unhandledrejection event

Baseline Widely available

This feature is well established and works across many devices and browser versions. Its been available across browsers since July 2020.

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Note: This feature is only available in Web Workers.

The unhandledrejection event is sent to the global scope (typically WorkerGlobalScope) of a script when a Promise that has no rejection handler is rejected.

This is useful for debugging and for providing fallback error handling for unexpected situations.

Syntax

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("unhandledrejection", (event) => { })

onunhandledrejection = (event) => { }

Event type

A PromiseRejectionEvent. Inherits from Event.

Event PromiseRejectionEvent

Examples

Basic error logging

This example logs information about the unhandled promise rejection to the console.

self.addEventListener("unhandledrejection", (event) => {
  console.warn(`UNHANDLED PROMISE REJECTION: ${event.reason}`);
});

You can also use the onunhandledrejection event handler property to set up the event listener:

self.onunhandledrejection = (event) => {
  console.warn(`UNHANDLED PROMISE REJECTION: ${event.reason}`);
};

Many environments (such as Node.js) report unhandled promise rejections to the console by default. You can prevent that from happening by adding a handler for unhandledrejection events thatin addition to any other tasks you wish to performcalls preventDefault() to cancel the event, preventing it from bubbling up to be handled by the runtime's logging code. This works because unhandledrejection is cancelable.

self.addEventListener("unhandledrejection", (event) => {
  // code for handling the unhandled rejection
  // 

  // Prevent the default handling (such as outputting the
  // error to the console)

  event.preventDefault();
});