The `onunhandledrejection` is an event handler to process `unhandledrejection` events that occur whenever a `Promise` rejection is not handled.
Before going deep into `onunhandledrejection`, let's learn about `Promise` rejection.

#   

## The `Promise` rejection

`Promise` is an object of JavaScript, which is responsible for handling asynchronous events for either a **completed state** or a **rejected state**, which are called `resolve` and `reject`, respectively. Initially, it possesses a **pending state**, then goes to a completion or rejection state, depending on the outcome.

#   
## Code example


The method `reject(reason)` of the `Promise` object is used to return the rejected `Promise` with a reason for the rejection in parameters.

Look at the below code:

// Rejecting the promise with a reason
var promise = Promise.reject("Oops! I am rejected");

promise.then(() => {
    console.log("Hello World");
});

#   
#  Code explanation
In the above code, the `promise.then()` function deals with the asynchronous calls in JavaScript. The `Promise` object is rejected with the reason `"Oops! I am rejected"` but there is nothing to catch this rejection, so this is an unhandled rejection. It will directly go to the console, and an error message will be shown like this:

`(node:22) UnhandledPromiseRejectionWarning: Oops! I am rejected`

and

`(node:22) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block or by rejecting a promise which was not handled with .catch(). (rejection id: 2)`

These errors show that an unhandled promise rejection is thrown by the code and not caught.

> **Note:** A depreciation warning `(node:21) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated.` will also be displayed because unhandled promise rejections are deprecated now.

Now, we will use the `onunhandledrejection` event handler to catch the rejection thrown.
#   

## The `onunhandledrejection` event handler

To catch the error gracefully and show it on the console, we can use a different method like `catch` or `onunhandledrejection`, but here, we are going to catch the `Promise` rejection using the globally available `onunhandledrejection` property from the `window` object. This is useful for debugging and error handling for unexpected cases.
##  
## Code Example

var promise = Promise.reject("Oops! I am rejected");

promise.then(() => {
    console.log("Hello World");
});

window.onunhandledrejection = (event) => {
    console.log("Unhandled Promise rejection caught here! Rejection reason: ", event.reason);
}

codemirror

console

#   
## Code explanation
In the above code, we have added `window.onunhandledrejection` with a message. This will catch the rejection thrown by `Promise.reject()` and display the message gracefully. The following output will be shown:

What is an onunhandledrejection in javascript?

The Promise rejection

Code example

Code explanation

The onunhandledrejection event handler

Code Example

Code explanation

The `Promise` rejection

The `onunhandledrejection` event handler