javascript - 稍后捕获 Promise 拒绝

Question

这个问题在这里已经有了答案:

Waiting for more than one concurrent await operation (4 个回答)

关闭 3年前。

我如何在以后检索 promise 的结果？在测试中，我在发送更多请求之前检索了一封电子邮件:

const email = await get_email();
assert.equal(email.subject, 'foobar');
await send_request1();
await send_request2();

在缓慢的电子邮件检索过程中如何发送请求？

起初，我考虑过等一下邮件:

// This code is wrong - do not copy!
const email_promise = get_email();
await send_request1();
await send_request2();
const email = await email_promise;
assert.equal(email.subject, 'foobar');

这在 get_email() 成功时有效，但如果 get_email() 在相应的 await 之前失败，则失败，并带有完全合理的 UnhandledPromiseRejectionWarning.

当然，我可以使用 Promise.all，像这样:

await Promise.all([
    async () => {
        const email = await get_email();
        assert.equal(email.subject, 'foobar');
    },
    async () => {
        await send_request1();
        await send_request2();
    },
]);

但是，它使代码更难阅读(它看起来更像基于回调的编程)，特别是如果以后的请求实际上依赖于电子邮件，或者存在一些嵌套。是否可以存储 promise 的结果/异常并在以后 await 它？

如果需要，here is a testcase模拟有时会失败，有时会工作，时间随机。它绝不能输出 UnhandledPromiseRejectionWarning。

Answer 1

const wait = (ms) => new Promise(resolve => setTimeout(resolve, ms));
const send_request1 = () => wait(300), send_request2 = () => wait(200);
async function get_email() {
    await wait(Math.random() * 1000);
    if (Math.random() > 0.5) throw new Error('failure');
    return {subject: 'foobar'};
}

const assert = require('assert');
async function main() {
    // catch possible error
    const email_promise = get_email().catch(e => e);
    await send_request1();
    await send_request2();
    // wait for result
    const email = await email_promise;
    // rethrow eventual error or do whatever you want with it
    if(email instanceof Error) {
      throw email;
    }
    assert.equal(email.subject, 'foobar');
};

(async () => {
    try {
        await main();
    } catch(e) {
        console.log('main error: ' + e.stack);
    }
})();