Promise 的第二个 .then() 没有失败

问题描述

我在链接 .then() 对承诺的调用时遇到问题.执行以下代码时:

I'm having trouble with chaining .then() calls for a promise. When executing the following code:

var prom = new Promise(function(resolve, reject) {
  //let's always fail
  reject(Error("buuuu!"));
});

var thenable =
prom.then(
    function(done) {
        console.log("First handler: Done!: Argument: ", done);
        return "First then: DONE";
    },
    function(fail) {
        console.error("First handler: Fail!. Argument: ", fail);
        return "First then: FAIL";
    }
).then(
    function(done) {
        console.info("Second handler: Done!.  Argument: ", done);
    },
    function(fail) {
        console.error("Second handler: Fail!.  Argument: ", fail);
    }
);

这将在控制台中打印以下内容:

This prints the following in the console:

First handler: Fail!. Argument:  Error {stack: (...), message: "buuuu!"}
Second handler: Done!.  Argument:  First then: FAIL

为什么第二个 then() 调用了它的 done 处理程序而不是 fail 一个?

Why does the second then() has its done handler called instead of the fail one?

这是 Chrome 的错误吗?(请注意，我只对 Chrome 浏览器的行为感兴趣)

Is this a bug with Chrome? (Note that I'm only interested in Google Chrome's behavior)

我是否需要从 .then 处理程序返回预先解析/拒绝的承诺?

Do I need to resort to returning pre-resolved/rejected Promises from the .then handlers?

推荐答案

因为您已经在链的第一个 then() 中处理了错误，所以使用您从返回的 "First then: FAIL" 字符串解析承诺

Because you handled the error in the first then() of the chain already, making the promise resolve with the "First then: FAIL" string that you returned from it.

这是 Chrome 的错误吗?

不，这就是 Promise 的工作方式.

No, that's how promises are supposed to work.

我是否需要从 .then 处理程序返回预先解析/拒绝的承诺?

你可以这样做，是的.触发第二个失败处理程序的其他方法是:

You can do that, yes. Other ways to trigger the second fail handler would be:

• 只需省略第一个错误处理程序:

• Simply omit the first error handler:

prom.then(function(done) {
    console.log("First handler: Done!: Argument: ", done);
    return "First then: DONE";
}).then(function(done) {
    console.info("Second handler: Done!.  Argument: ", done);
}, function(fail) {
    console.error("Second handler: Fail!.  Argument: ", fail);
});

或者重新抛出异常(类似于返回被拒绝的承诺):

Or rethrow an exception (that's similar to returning a rejected promise):

prom.then(function(done) {
    console.log("First handler: Done!: Argument: ", done);
    return "First then: DONE";
}, function(fail) {
    console.error("First handler: Fail!. Argument: ", fail);
    throw new Error("First then: FAIL"); // or: throw fail;
    // alternatively, you can return a rejected promise:
    return Promise.reject(new Error("First then: FAIL"));
}).then(function(done) {
    console.info("Second handler: Done!.  Argument: ", done);
}, function(fail) {
    console.error("Second handler: Fail!.  Argument: ", fail);
});

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