I'm using nock.back to mock out some API calls. When unexpected calls are made, UnhandledPromiseRejectionWarning gets printed to the console, but my tests pass, and those warnings are easily missed in the rest of the console output. I want an exception thrown instead of a silent error. How do I do this?
The way I use promises is:
function myFunction(){
return new Promise((resolve, reject) -> {
try {
// Logic
resolve(logic)
} catch(e) {
reject('Provide your error message here -> ' + e)
}
})
}
Or !
function myFunction().then( // Calls the function defined previously
logic => { // Instead of logic you can write any other suitable variable name for success
console.log('Success case')
},
error => {
console.log('myFunction() returned an error: ' + error)
}
)
UPD
Have you had a look here? https://nodejs.org/api/process.html#process_event_unhandledrejection
It describes an unhandledRejection event being emitted when you have no catch for a rejection from a Promise and provides the code to catch the WARNING and output it nicely to console.
(copy paste)
process.on('unhandledRejection', (reason, p) => {
console.log('Unhandled Rejection at:', p, 'reason:', reason);
// application specific logging, throwing an error, or other logic here
});
Node.js runs on a single process.
Related
I'm using rxjs in a library to expose an Observable that clients can subscribe to to consume messages. I want to be able to react appropriately if the subscriber's next function throws an error. However, I'm not seeing any obvious way to detect that. For example:
const observable = new Observable<string>((subscriber) => {
subscriber.next('first')
subscriber.next('second')
subscriber.complete()
})
observable.subscribe(() => {
throw new Error('oh no!')
})
I have tried all of the following, but the errors are bubbled all the way up to a global scope that's surfaced either in an onUnhandledError function provided to the global config, or in absence of that, the node process's unhandledException event.
process.on('uncaughtException', (error) => {
console.error('IN UNCAUGHT EXCEPTION HANDLER', error.message)
})
export function main() {
try {
const observable = new Observable<string>((subscriber) => {
try {
subscriber.next('first')
} catch (error) {
console.error('IN OBSERVABLE CATCH', error.message)
}
subscriber.complete()
}).pipe(
catchError((error) => {
console.error('CATCHERROR PIPE', error.message)
return of('there was an error!!!!')
}),
)
observable.subscribe({
next: (_value) => {
throw new Error('oh no!')
},
error: (error) => {
console.error('IN OBSERVER ERROR HANDLER', error.message)
},
complete: () => console.log('complete!'),
})
} catch (error) {
console.error('IN MAIN CATCH', error.message)
}
}
This logs:
complete!
IN UNCAUGHT EXCEPTION HANDLER oh no!
The docs don't make a big fuss about ensuring that subscribers avoid throwing errors at all costs, but I don't see a standard mechanism for handling it short of some sort of "observer wrapper" (that gets a bit ugly with the overloads).
Turns out that there is effectively no way to handle errors from observers.
When using Observable.subscribe, it wraps your observer functions in a "SafeSubscriber". This then wraps the supplied functions with a ConsumerObserver. This wraps each in a try/catch that, upon errors, either:
sends them to an optional onUnhandledError function you can supply to the rxjs config
sends them "into the ether" to be picked up by the node process
There is no context, and no way to hook into it. Effectively, errors from next, error, or complete handlers just silently disappear.
I want to continue code execution when my promises do not have errors, If there are any errors however, I do not want to proceed. How do I gracefully handle this case ?
Eg.
/* start of REST call */
const resultA = fetchThings()
.then(things => returnSomeResult(things))
.then(...)
...
.catch(err => throw err); // cannot proceed computation if there is error
const resultB = fetchOtherThings()
.then(otherTings => returnOtherResult(otherThings))
.then(...)
...
.catch(err => throw err); // cannot proceed if there is error
const resultC = Promise.all([resultA, resultB]) => { // do other stuff }
// return resultC from REST call if successful
When an error occurs in either of the results, I am getting a UnhandledPromiseRejectionWarning on the terminal. It states that 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 a .catch(). I suspect it is because I am throwing the error inside the catch block.
However, I don't want the code to proceed when there is an error from either resultA or resultB.
How can I better handle this scenario of mine ?
My guess would be that you're not handling errors thrown by Promise.all(), which will be rejected if any of the promises it's handling will be rejected.
So you need to catch errors there as well:
Promise.all(…).then(…).catch(e => …)
FWIW, your catch()'s are superfluous:
.catch(err => throw err)
You're catching errors that were thrown and rethrowing them, which isn't necessary (they will be caught by .catch() that you'll attach to Promise.all()).
My system consists of an Angular UI and a Node API. The UI submits a file to the API for processing, then gets the result back. This all works - however - the API sometimes fails at processing unexpected data.
I want to be able to catch the error(s) when they arise, stop execution so they won't screw up the UI, then send a message back to UI.
Here is my code so far:
const IncomingForm = require('formidable').IncomingForm;
asynch function myApi(req, res)
{
try // (1)
{
var form = new IncomingForm(...);
form.on('file', async(field, file) =>
{
const [result] = await sometimesBad(inParam); // (2) attach .catch(...);
...
res.send({goodFinal}); // execution should not reach here if error occurs before
});
form.on('end', ()=> {})
form.parse(req)
}
catch (erApi) // (3)
{
... // (4)
}
}
async function sometimesBad(x)
{
try // (5)
{
... lines of code could have run-time error depends on x ...
return goodResult;
}
catch(err2) // (6)
{
... // (7)
}
}
Currently, after hours of searching and trial and error, I:
am able to send a message back by chaining a .catch() at (2)
am unable to stop the execution via any combination of (1), (3), (4), (5), (6), (7), including the use of next(), throw new Error(), await Promise.reject(), return Promise.reject().
am always getting UnhandledPromiseRejectionWarning: Unhandled promise rejection.
Node version: 14.9
Update: In addition to accepted answer, there is no need to have (5), (6), (7).
In your code if (2) throws the error indeed is not handled. To handle it, you need to wrap the code inside async (field, file) => ... into try / catch, similar to how you did on the top level of middleware, and inside the catch you do next(error). Also add default error handler after all routes in your app. See How to return error to the client client without making node server crash? regarding that.
You can stop unhandledRejection(s) from crashing your app by handling them. However, if you fail to handle them using catch blocks, you can also watch for events on the process.
Code Example from the Docs:
process.on('unhandledRejection', (reason, promise) => {
console.log('Unhandled Rejection at:', promise, 'reason:', reason);
// Application specific logging, throwing an error, or other logic here
});
somePromise.then((res) => {
return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`
Alternatively, you can make your sometimesBad function return a Promise, which would cause all errors happening inside the Promise body to be thrown, which can then be handled in the catch block of the caller.
I have a code like this (simplified):
getStreamFor(path) {
// both, remote and local, return a Promise
if(...) { return getRemoteFileStream(path); }
else { return getLocalFileStream(path); }
}
getRemoteFileStream(path) {
// should throw in my case (MyError)
const validPath = validatePath(path);
return readStreamIfValid(validPath);
}
and in the test case:
it('should throw MyError', () => {
return getStreamFor(path)
.then(() => {})
.catch(error => expect(error).to.be.instanceOf(MyError));
});
The problem is, that when the validatePath(path) Method throws (due to invalid path), nothing get caught in the test case promise. The output in my terminal / console is a regular exception as if it was uncaught.
Does anybody have an idea, why the the Promise wouldn't recognize the throw? How can I fix it without probably surrounding the call in the test case with another "try catch" (since the promise should do that for me)?
Maybe there is a general best practise how to structure Promises in order to avoid error swallowings like these?
Thanks for your help!
The problem here is that validatePath() is not part of the promise chain returned by getRemoteFileStream()
One possible solution is the following:
getRemoteFileStream(path) {
return Promise.resolve()
.then(() => validatePath(path))
.then(validPath => readStreamIfValid(validPath));
}
An exception thrown by validatePath() would now be handled in the Promise's catch handler.
I have a Express router with specific request handler:
router.post(def.doc_create, handle);
async function docCreate(req, res) {
let metadata = mapper.mapObject(req.body, templates.document_create_rq);
let relatedVers = mapper.mapArray('dms$relatedDocuments', templates.related_doc_vers, req.body).items;
// if (req.body['dms$mchMetadata']) {
try {
let productInstanceIds = req.body['dms$mchMetadata']["pro$productInstanceIds"] || [];
} catch (err) {
throw new Error(err.message);
}
....
}
'safeHandleReq' method will pick up the right handler for given route from the pre-initialised map and will call it:
class SafeRequestHandler {
constructor(reqMappings) {
this.mappings = reqMappings;
}
safeHandleReq(req, res) {
try {
let pair = _.find(this.mappings, {'url': req.url});
return pair.handler(req, res);
} catch (err) {
return sendEnhaced(err, res);
}
}
}
with this handler being executed in try-catch I wanted to avoid UnhandledPromiseRejectionWarning: Unhandled promise rejection errors. This happens for example, where there is no such property in req.body that I'm asking for. Now exactly this happens in try-catch block I have shown here. The request's body does not contain dms$mchMetadata property at all. Now I have expected the catch block of the handler to be hit. But it's not. Even when I re-throw caught error in the 'docCreate' handler itself, safeHandleReq's handler catch block won't catch the error, but forementioned error is being written on the console instead.
I need to be able to catch all sorts of this errors in handler's catch block because there is many places where anticipations can go wrong and I need to return (somethong). When the error is not caught execution hangs and Express won't respond.
So why and what I need to do in order to be able to catch all errors from handler implementation in the safeHandleReq try-catch block? I need a systematic solution.
Thanks for recommendations!