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Promise - unhandled rejection warning has been thrown, although the rejection seems to be handled #1208
Description
- Node.js Version: v9.11.1
- OS: windows 10
- Scope (install, code, runtime, meta, other?): ?
- Module (and version) (if relevant): ???
We have a code like this
'use strict';
var a = new Promise((res, rej) => {
rej(123);
console.log('not ok');
});
a.then(() => console.log(1));
a.catch(a => console.log(a));The output is following:
> node example.js
not ok
123
(node:4748) UnhandledPromiseRejectionWarning: 123
(node:4748) 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:4748) [DEP0018] DeprecationWarning: Unhandled promise rejections are depre
cated. In the future, promise rejections that are not handled will terminate the
Node.js process with a non-zero exit code.So we are confused about why this happens:
- Why is
not okprinted before123? - Why do we get unhandled warning while the rejection is handled (it has catch properly attached)?
- How to do it properly (to avoid warning and to reject silently)?
Sorry if this question is noobish, can't figure out how to do this for a week. Tried stack overflow, but got no response and no votes at all, the post is ignored completely
Activity
ryzokuken commentedon Apr 11, 2018
@user8322 no question is noobish, and if it's troubling you then you should ask out without thinking too much about it :)
Regarding 2 and 3, the unhandled warning came up because it was, in fact, not handled properly, because you tried handling success and failure at different statements. The correct code would be:
and indeed, this work. See for yourself.
Regarding 1, this happens because of how Node.js works and it's internals (as well as v8's). To get started with promises (regarding syntax and internals), try out nodeschool's workshop, "Promise it won't hurt" at https://github.com/stevekane/promise-it-wont-hurt