Baseline Widely available
The finally() method of Promise instances schedules a function to be called when the promise is settled (either fulfilled or rejected). It immediately returns another Promise object, allowing you to chain calls to other promise methods.
This lets you avoid duplicating code in both the promise's then() and catch() handlers.
function checkMail() {
return new Promise((resolve, reject) => {
if (Math.random() > 0.5) {
resolve("Mail has arrived");
} else {
reject(new Error("Failed to arrive"));
}
});
}
checkMail()
.then((mail) => {
console.log(mail);
})
.catch((err) => {
console.error(err);
})
.finally(() => {
console.log("Experiment completed");
});
promiseInstance.finally(onFinally)
onFinally
A function to asynchronously execute when this promise becomes settled. Its return value is ignored unless the returned value is a rejected promise. The function is called with no arguments.
Returns a new Promise immediately. This new promise is always pending when returned, regardless of the current promise's status. If onFinally throws an error or returns a rejected promise, the new promise will reject with that value. Otherwise, the new promise will settle with the same state as the current promise.
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 then(onFinally, onFinally). However, there are a couple of differences:
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.finally() call is usually transparent and reflects the eventual state of the original promise. So for example:
Promise.resolve(2).then(() => 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.Promise.reject(3).then(() => {}, () => 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 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 â which, for 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.
promise.then(
(value) => Promise.resolve(onFinally()).then(() => value),
(reason) =>
Promise.resolve(onFinally()).then(() => {
throw reason;
}),
);
Because finally() calls then(), it supports subclassing. Moreover, notice the Promise.resolve() call above â 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].
let isLoading = true;
fetch(myRequest)
.then((response) => {
const contentType = response.headers.get("content-type");
if (contentType && contentType.includes("application/json")) {
return response.json();
}
throw new TypeError("Oops, we haven't got JSON!");
})
.then((json) => {
/* process your JSON further */
})
.catch((error) => {
console.error(error); // this line can also throw, e.g. when console = {}
})
.finally(() => {
isLoading = false;
});
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