Promise.prototype.finally()
The finally()
method returns a Promise
.
When the promise is finally either fulfilled or rejected, the specified callback
function is executed. This provides a way for code to be run whether the promise was
fulfilled successfully, or instead rejected.
This helps to avoid duplicating code in both the promise's then()
and catch()
handlers.
Syntax
p.finally(onFinally);
p.finally(function() {
// settled (fulfilled or rejected)
});
Parameters
onFinally
-
A
Function
called when thePromise
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 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
-
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. So for example:-
Unlike
Promise.resolve(2).then(() => {}, () => {})
(which will be resolved withundefined
),Promise.resolve(2).finally(() => {})
will be resolved with2
. -
Similarly, unlike
Promise.reject(3).then(() => {}, () => {})
(which will be fulfilled withundefined
),Promise.reject(3).finally(() => {})
will be rejected with3
.
-
Unlike
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
Using finally
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.error(error); /* this line can also throw, e.g. when console = {} */ })
.finally(function() { isLoading = false; });
Specifications
Specification |
---|
ECMAScript Language Specification # sec-promise.prototype.finally |
Browser compatibility
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