javascript - setInterval 和 promises 产生一个 PromiseRejectionHandledWarning

Question

我有一个应用程序，我必须不断地在后台运行一些异步代码。我创建了我的应用程序的最小模拟。

let promise_chain = Promise.resolve();
let rejected_promise_count = 0;

const interval_id = setInterval(
    // Do some important polling. I will just always reject to demonstrate the problem.
    () => {
        promise_chain = promise_chain.then(() => {
            rejected_promise_count += 1;
            return Promise.reject();
        })
    },
    10
);

// Set timeout simulates the program being done.
setTimeout(
    () => {
        clearInterval(interval_id);
        promise_chain
            .then(() => end("Resolved! :D"))
            .catch(() => end("Rejected! D:"));
    },
    1000
);

function end(message) {
    console.log(message);
    console.log(`Amount of rejected promises created: `, rejected_promise_count);
}

这给出了一长串这些:

(node:29217) UnhandledPromiseRejectionWarning: undefined
(node:29217) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:29217) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:29217) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 2)
(node:29217) UnhandledPromiseRejectionWarning: undefined
(node:29217) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
(node:29217) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 3)
(node:29217) UnhandledPromiseRejectionWarning: undefined
(node:29217) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 4)
(node:29217) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 4)

最终以这些结束:

(node:29217) UnhandledPromiseRejectionWarning: undefined
(node:29217) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 87)
(node:29217) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 87)
(node:29217) UnhandledPromiseRejectionWarning: undefined
(node:29217) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 88)
Rejected! D:
Amount of rejected promises created:  1
(node:30920) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 89)

我不太清楚为什么 node 不希望我异步处理这些，但考虑到它按时间间隔运行，我真的别无选择。

我如何摆脱无穷无尽的警告列表，更重要的是，我如何确保将来 Node 不会因为它认为我没有处理拒绝而终止此进程？

Answer 1

这取决于你想做什么，正确的处理方式是这样的:

const interval_id = setInterval(
    () => {
        promise_chain = promise_chain.then(() => {
            rejected_promise_count += 1;
            return Promise.reject();
        });
        //ignore error here, you catch it in the setTimeout
        promise_chain.catch(ignore=>ignore);
    },
    10
);

这将输出:

Rejected! D:
Amount of rejected promises created:  1

这是因为你第一次拒绝所以链条被打破并且所有其他then都没有被执行。

如果您想继续执行并想知道有多少通过和失败，您可以这样做:

//using actual results
let results = [];
//special Fail value to indicate rejected promise
let Fail = function(reason){this.reason=reason;};
let isFail = object=>(object&&object.constructor===Fail);
let isNotFail = object=>!isFail(object);

let promise_chain;
let somePromise = ()=>Promise.reject(new Error("Some reason"));

const interval_id = setInterval(
    () => {
        promise_chain = (promise_chain||somePromise())
        .then(
          result => {
            results.push(result);
            return somePromise();
          }
        )
        .catch(//catch the rejection and return a Fail type value
          error=>new Fail(error)
        );
},
    10
);

// Set timeout simulates the program being done.
setTimeout(
    () => {
        clearInterval(interval_id);
        promise_chain
        .then(
          result => {
            //add the last result to results
            results.push(result);
            console.log(
              "rejected promises:",
              results.filter(isFail).length
              //you can get the errors like so:
              //results.filter(isFail).map(fail=>fail.reason)
            );
            console.log(
              "resolved promises:",
              results.filter(isNotFail).length
              //results.filter(isNotFail) is an array of resolved values
            )
          }
        );
            //this will never reject because we catch rejected promises
            //  and add fail types to results
            // .catch(() => end("Rejected! D:"));
    },
    1000
);