HTML unhandledrejection DOM Event: Reference and Examples

When someone interacts with a webpage, the browser sends events to the relevant JavaScript code. The unhandledrejection event happens when a rejected promise has no rejection handler when the browser reports it.

When unhandledrejection happens

A rejected promise has no rejection handler when the browser reports it.

You can listen for it using addEventListener(). That keeps event handling separate from HTML attributes such as onclick and works well with modern code organization.

A small working example

<p>Try the relevant browser action described below.</p>
<p id="event-status" role="status">Waiting for unhandledrejection...</p>
<script>
    const eventTarget = window;
    const eventStatus = document.getElementById("event-status");

    if (eventTarget) {
        eventTarget.addEventListener("unhandledrejection", (eventDetails) => {
            eventStatus.textContent = "unhandledrejection event received";
            console.log(eventDetails.type, eventDetails.target);
        });
    }
</script>

Reading the code

The browser listens for unhandledrejection on the relevant target. When the event occurs, the callback receives eventDetails, updates the status message and logs the event name and target in Developer Tools.

In a safe demo reject a promise without a catch; do not use this instead of proper error handling.

eventDetails.reason contains the rejection reason; catch expected errors where they happen.

Some events need a specific browser condition, another tab or window, or a Web Component to reproduce. Read the trigger steps carefully before deciding that an event handler is not working.

Try it in your browser

Open the page, show the Console panel, and follow this test: In a safe demo reject a promise without a catch; do not use this instead of proper error handling. Then change one thing in the example and observe whether the event still fires.

Common Beginner Mistakes

• Expecting unhandledrejection to fire without its required user action or browser condition.
• In a safe demo reject a promise without a catch; do not use this instead of proper error handling.
• Adding repeated event listeners without considering cleanup or unwanted side effects.

Key Takeaways

• unhandledrejection reports a rejected promise has no rejection handler when the browser reports it.
• addEventListener("unhandledrejection", ...) listens for the event on the appropriate target.
• In a safe demo reject a promise without a catch; do not use this instead of proper error handling.