Promise.prototype.finally()

finally () gibt ein Promise zurück. Wenn das Promise erfüllt oder abgelehnt wird, wird die angegebene callback-Funktion ausgeführt. Dies stellt eine Möglichkeit für Code bereit, der ausgeführt werden muss, unabhängig davon, ob das Promise erfolgreich erfüllt wurde oder nicht. Auf diese Weise können Sie vermeiden, dass derselbe Code für die beiden Handlern des Promise then () und catch () geschrieben werden muss.

Syntax

p.finally(onFinally);

p.finally(function() {
   // settled (fulfilled or rejected)
});

Parameters

onFinally
A Function called when the Promise is settled.

Return value

Returns a Promise whose finally handler is set to the specified function, onFinally.

Description

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 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
  • A finally callback will not receive any argument, since there's no reliable means of determining if the promise was fulfilled or rejected. 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.
  • Unlike Promise.resolve(2).then(() => {}, () => {}) (which will be resolved with undefined), Promise.resolve(2).finally(() => {}) will be resolved with 2.
  • Similarly, unlike Promise.reject(3).then(() => {}, () => {}) (which will be fulfilled with undefined), Promise.reject(3).finally(() => {}) will be rejected with 3.

Note: A throw (or returning a rejected promise) in the finally callback will reject the new promise with the rejection reason specified when calling throw().

Examples

let isLoading = true;

fetch(myRequest).then(function(response) {
    var 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(function(json) { /* process your JSON further */ })
  .catch(function(error) { console.log(error); })
  .finally(function() { isLoading = false; });

Specifications

Specification Status Comment
TC39 proposal Stage 4

Browser compatibility

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See also