Promise.prototype.finally()

¥Syntax

promiseInstance.finally(onFinally)

¥Parameters

onFinally

当此承诺得到解决时异步执行的函数。除非返回值是被拒绝的 Promise，否则它的返回值将被忽略。调用该函数时不带参数。

¥Return value

立即返回新的 Promise。无论当前 Promise 的状态如何，该新 Promise 在返回时始终处于待处理状态。如果 onFinally 抛出错误或返回被拒绝的 Promise，则新的 Promise 将以该值拒绝。否则，新的 Promise 将以与当前 Promise 相同的状态进行结算。

¥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.

¥Description

如果你想要在承诺解决后进行一些处理或清理（无论其结果如何），则 finally() 方法可能很有用。

¥The finally() method can be useful if you want to do some processing or cleanup once the promise is settled, regardless of its outcome.

finally() 方法与调用 then(onFinally, onFinally) 非常相似。但是，存在一些差异：

¥The finally() method is very similar to calling then(onFinally, onFinally). However, there are a couple of differences:

• 创建内联函数时，你可以传递一次，而不是被迫声明两次或为其创建一个变量。
• onFinally 回调不接收任何参数。此用例恰好适用于你不关心拒绝原因或履行价值的情况，因此无需提供它。
• finally() 调用通常是透明的，不会更改原始 Promise 的最终状态。例如：
  • 与 Promise.resolve(2).then(() => 77, () => {}) 返回最终用值 77 实现的承诺不同，Promise.resolve(2).finally(() => 77) 返回最终用值 2 实现的承诺。
  • 类似地，与 Promise.reject(3).then(() => {}, () => 88) 返回一个最终以 88 值实现的 Promise 不同，Promise.reject(3).finally(() => 88) 返回一个最终以 3 为理由拒绝的 Promise。

注意：finally 回调中的 throw （或返回被拒绝的 Promise）仍然会拒绝返回的 Promise。例如，Promise.reject(3).finally(() => { throw 99; }) 和 Promise.reject(3).finally(() => Promise.reject(99)) 都拒绝返回的 promise，原因为 99。

¥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.

与 catch() 一样，finally() 在调用它的对象上内部调用 then 方法。如果 onFinally 不是函数，则使用 onFinally 作为两个参数来调用 then() — 对于 Promise.prototype.then()，这意味着没有附加有用的处理程序。否则，将使用两个内部创建的函数调用 then()，其行为如下：

¥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 — 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:

警告：这仅用于演示目的，而不是填充。

¥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;
    }),
);

因为 finally() 调用 then()，所以它支持子类化。此外，请注意上面的 Promise.resolve() 调用 - 实际上，onFinally() 的返回值是使用与 Promise.resolve() 相同的算法解析的，但用于构造已解析 Promise 的实际构造函数将是子类。finally() 通过 promise.constructor[@@species] 得到这个构造函数。

¥Because finally() calls then(), it supports subclassing. Moreover, notice the 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[@@species].

¥Examples

¥Using finally()

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;
  });

¥See also

• core-js 中 Promise.prototype.finally 的 Polyfill
• Promise
• Promise.prototype.then()
• Promise.prototype.catch()