我如何捕获ES6 Promise拒绝并完全停止流动？

问题描述

假设我有4个函数： runA（）， runB（）， runC （）和 runD（）。

Say I have 4 functions: runA(), runB(), runC() and runD().

使用ES6承诺，完全成功运行，这些都将一个接一个地运行：

Using ES6 promises, in a completely successful run, these would all be run one after another:

runA()
.then(runB)
.then(runC)
.then(runD)

如果 runA 或 runB 失败（拒绝或抛出），我想调用 error1（）然后完全停止链（不是调用 runC 或 runD ）。这让我觉得我应该在 .then promise链的最后添加一个 .catch（）：

If runA or runB fail (reject or throw), I would like to call error1() and then completely stop the chain (not call runC or runD ). This makes me think I should add a single .catch() at the very end of the .then promise chain:

runA()
.then(runB)
.then(runC)     //won't get called if runA or runB throws
.then(runD)     //won't get called if runA or runB throws
.catch(error1)

但如果 runC 失败，我想调用 error2（）并且仍然停止链（不是致电 runD ）。

But if runC fails, I would like to call error2() and still stop the chain (not call runD).

runA()
.then(runB)   
.catch(error1)  //moved up to only handle runA and runB
.then(runC)     //but now this gets called after error1() is run
.then(runD)     
.catch(error2)

现在我在链中有2个 catch 调用， runC 将在 error1 正在运行，因为catch的结果将默认为 resolve 。我唯一的选择是让 error1 函数创建一个它总是拒绝的承诺吗？

Now that I have 2 catch calls in the chain, runC will get called after error1 is run since the result of the catch will default to a resolve. Is my only option to have the error1 function create a promise that it always rejects?

推荐答案

不，让 error1 创建一个始终拒绝的承诺， 不 是您唯一的选择。

No, having error1 create a promise that always rejects, is not your only option.

您可以利用 .then 接受两个参数的事实：

You can exploit the fact that .then takes two arguments:

.then(onSuccess, onFailure)

给出两个论点， onFailure 在中捕获失败的效果不明显的onSuccess 。这通常是不受欢迎的，除了在这里，您可以使用此事实来分支您的决策树：

When given two arguments, there's an under-appreciated effect that onFailure will not catch failures in onSuccess. This is usually undesirable, except here, where you can use this fact to branch your decision tree:

runA()
.then(runB)
.then(() => runC().then(runD), error1)
.catch(error2)

这可以满足您的需求。

• if runA 或 runB 失败，然后调用 error1 并停止链。


• 如果 runC 或 runD 失败，则 error2 被调用并且链停止。

• if runA or runB fail, then error1 is called and chain stops.
• if runC or runD fail, then error2 is called and chain stops.

您也可以这样写：

runA()
.then(runB)
.then(() => runC()
  .then(runD)
  .catch(error2),
error1)

var log = msg => div.innerHTML += "<br>" + msg;

// Change which one of these four rejects, to see behavior:
var runA = () => Promise.resolve().then(() => log("a"));
var runB = () => Promise.reject().then(() => log("b"));
var runC = () => Promise.resolve().then(() => log("c"));
var runD = () => Promise.resolve().then(() => log("d"));
var error1 = () => log("error1");
var error2 = () => log("error2");

runA()
.then(runB)
.then(() => runC().then(runD), error1)
.catch(error2)

<div id="div"></div>

尝试修改哪一个失败。

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