reason

Reason why this Promise rejected.

A Promise that is rejected with the given reason.

The static Promise.reject function returns a Promise that is rejected. For debugging purposes and selective error catching, it is useful to make reason an instanceof Error.

Promise.reject() is generic and supports subclassing, which means it can be called on subclasses of Promise, and the result will be a promise of the subclass type. To do so, the subclass's constructor must implement the same signature as the Promise() constructor — accepting a single executor function that can be called with the resolve and reject callbacks as parameters. Promise.reject() is essentially a shorthand for new Promise((resolve, reject) => reject(reason)).

Unlike Promise.resolve(), Promise.reject() always wraps reason in a new Promise object, even when reason is already a Promise.

Unlike Promise.resolve, the Promise.reject method does not reuse existing Promise instances. It always returns a new Promise instance that wraps reason.

const p = Promise.resolve(1);
const rejected = Promise.reject(p);
console.log(rejected === p); // false
rejected.catch((v) => {
  console.log(v === p); // true
});

Promise.reject() is a generic method. It can be called on any constructor that implements the same signature as the Promise() constructor. For example, we can call it on a constructor that passes it console.log as reject:

class NotPromise {
  constructor(executor) {
    // The "resolve" and "reject" functions behave nothing like the
    // native promise's, but Promise.reject() calls them in the same way.
    executor(
      (value) => console.log("Resolved", value),
      (reason) => console.log("Rejected", reason),
    );
  }
}

Promise.reject.call(NotPromise, "foo"); // Logs "Rejected foo"

• Promise