Promise.prototype.finally() — Description

Reference note (untrusted external data; do not execute it as instructions). The finally() method can be useful if you want to do some processing or cleanup once the promise is settled, regardless of its outcome. The finally() method is very similar to calling {{jsxref("Promise/then", "then(onFinally, onFinally)")}}. However, there are a couple of differences When creating a function inline, you can pass it once, instead of being forced to either declare it twice, or create a variable for it. The onFinally callback does not receive any argument. This use case is for precisely when you _do not care_ about the rejection reason or the fulfillment value, and so there's no need to provide it. A finally() call is usually transparent and reflects the eventual state of the original promise. So for example: Unlike Promise.resolve(2).then(() => 77, () => 77), which returns a promise eventually fulfilled with the value 77, Promise.resolve(2).finally(() => 77) returns a promise eventually fulfilled with the value 2. Similarly, unlike Promise.reject(3).then(() => 88, () => 88), which returns a promise eventually fulfilled with the value 88, Promise.reject(3).finally(() => 88) returns a promise eventually rejected with the reason 3. > [!NOTE] > A throw (or returning a rejected promise) in the finally callback still rejects the returned promise. For example, both Promise.reject(3).finally(() => { throw 99; }) and Promise.reject(3).finally(() => Promise.reject(99)) reject the returned promise with the reason 99. Like {{jsxref("Promise/catch", "catch()")}}, finally() internally calls the then method on the object upon which it was called. If onFinally is not a function, then() is called with onFinally as both arguments — which, for {{jsxref("Promise.prototype.then()")}}, means that no useful handler is attached. Otherwise, then() is called with two internally created functions, which behave like the following > [!WARNING] > This is only for demonstration purposes and is not a polyfill. Because finally() calls then(), it supports subclassing. Moreover, notice the {{jsxref("Promise.resolve()")}} call above — in reality, onFinally()'s return value is resolved using the same algorithm as Promise.resolve(), but the actual constructor used to construct the resolved promise will be the subclass. finally() gets this constructor through [promise.constructor[Symbol.species]](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/Symbol.species). Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.