Catching errors with Q.deferred - javascript

I have a nodeJS project where I wish to use asynchronous functions. Namely, I'm using the Q library.
I have a function called someFunction(), that I wish to return a promise. With the then -function, I can then check whether the promise was resolved or rejected like so:
someFunction()
.then(
function(results) {
console.log("Success!");
},
function (error) {
console.log("An error occurred, and I would wish to log it for example");
}
);
What I intuitively expect with the function above is, that the error function would catch all possible errors. So if some exception is raised inside the someFunction, I can rest assured, that the error function will be run (the second function after 'then'). But it seems this is not the case.
For example, let's say the someFunction would be defined as so:
function someFunction() {
var deferred = Q.defer();
throw new Error("Can't bar.");
deferred.resolve("success");
}
Now if I call the function someFunction() like done in the upper code block, it won't run the error function. Why is that? Isn't the partial point of the promise Q.deferred to catch errors? Why should I manually reject every error that happens? I know I could set the whole content of someFunction to try/catch clause, and then reject the deferred, but that feels so wrong! There must be a better way, and I know for sure some of you stackoverflowers know it!
With this information, I began to think about where the deferred.reject and deferred.resolve is even meant to be used? Is it even meant to catch exceptions? Should I really just go through all the error cases manually, and call the deferred.reject on them? I'm interested to hear how this should be handled professionally.

Q has specific function for success and error, so use:
deferred.reject("error");
intead of throwing an Exception.
Next thing is that someFunction must return promise to be used as You use it:
function someFunction() {
var deferred = Q.defer();
try{
//some Asynchronous code
deferred.resolve("success");
}catch(e){
deferred.reject(e.message);
}
return deffered.promise; //return promise to use then
}

Because Promises ain't magic. They don't somehow magically catch Errors. They catch them, because they wrap the calls to the callbacks in try..catch-blocks, to convert Errors into rejected Promises.
If you want an Error to be handled by the Promise-chain, well put the function-call into a promise-chain: Q.resolve().then(someFunction).then(...).
Now any synchronous Error occuring in someFunction can be handled in the following then's.
Btw: If you use Q.defer(), and you're not dealing with some callback-style API, you're doing it definitely wrong. Search for the Deferred-antipattern.

it won't run the error function. Why is that?
Because you synchronously threw an exception, instead of returning a promise. Which you never should.
Isn't the partial point of the promise Q.deferred to catch errors?
No. then and catch implicitly catch exceptions in their callbacks, defferreds don't - they're just a (deprecated) API to create promises.
Why should I manually reject every error that happens?
Because asynchronous errors are expected to be passed to callbacks anyway, instead of being thrown.
I know I could set the whole content of someFunction to try/catch clause, and then reject the deferred, but that feels so wrong! There must be a better way!
There is: the Q.Promise constructor, the standard (ES6) promise creation API. It has the benefit of being able to catch synchronous exceptions from the executor function:
function someFunction() {
return new Q.Promise(function(resolve) {
throw new Error("Can't bar.");
resolve("success");
});
}

throwing an error will stop the code from executing (and will close node) unless you catch the error in a try/catch block.
handled errors from requests can be passed to the .catch chain by using deferred.reject(error). code errors and custom thrown errors needs to be handled inside try/catch, which is the right way to handle such errors.
function someFunction() {
var deferred = Q.defer();
deferred.reject("Can't bar.");
// or
try {
throw new Error("Can't bar.");
}
catch(err) {
deferred.reject("Can't bar.");
}
deferred.resolve("success");
}

Related

Javascript Try Catch vs Catch chain

I recently ran into a Javascript problem catching errors and thus crashing when exception thrown.
funcReturnPromise().then().catch()
I had to change this to:
try {
funcReturnPromise().then()
} catch (e) {
...
}
Couldn't find a decent explanation for it, any JS wizards available to enlighten a JS peasant?
If funcReturnPromise() can throw synchronously (which functions that return promises should generally never do), then you do have to catch that synchronous exception with try/catch as you discovered when using regular .then().
This is one place where async functions can hep you. For example, if you declare funcReturnPromise as async, then the synchronous exception it throws will automatically become a rejected promise and the caller won't ever be exposed to a synchronous exception.
Or, if the caller (your code here) uses await, then you can catch both sycnhronous exceptions and rejected promises with the same try/catch.
So, for example, you could do this:
async function myFunc()
try {
let result = await funcReturnPromise();
console.log(result);
} catch (e) {
// this catches both a rejected promise AND
// a synchronously thrown exception
console.log(e);
}
}

How to return from a Promise's catch/then block?

There are many tutorials on how to use "then" and "catch" while programming with JavaScript Promise. However, all these tutorials seem to miss an important point: returning from a then/catch block to break the Promise chain. Let's start with some synchronous code to illustrate this problem:
try {
someFunction();
} catch (err) {
if (!(err instanceof MyCustomError))
return -1;
}
someOtherFunction();
In essence, I am testing a caught error and if it's not the error I expect I will return to the caller otherwise the program continues. However, this logic will not work with Promise:
Promise.resolve(someFunction).then(function() {
console.log('someFunction should throw error');
return -2;
}).catch(function(err) {
if (err instanceof MyCustomError) {
return -1;
}
}).then(someOtherFunction);
This logic is used for some of my unit tests where I want a function to fail in a certain way. Even if I change the catch to a then block I am still not able to break a series of chained Promises because whatever is returned from the then/catch block will become a Promise that propagates along the chain.
I wonder if Promise is able to achieve this logic; if not, why? It's very strange to me that a Promise chain can never be broken. Thanks!
Edit on 08/16/2015:
According to the answers given so far, a rejected Promise returned by the then block will propagate through the Promise chain and skip all subsequent then blocks until is is caught (handled). This behavior is well understood because it simply mimics the following synchronous code (approach 1):
try {
Function1();
Function2();
Function3();
Function4();
} catch (err) {
// Assuming this err is thrown in Function1; Function2, Function3 and Function4 will not be executed
console.log(err);
}
However, what I was asking is the following scenario in synchronous code (approach 2):
try {
Function1();
} catch(err) {
console.log(err); // Function1's error
return -1; // return immediately
}
try {
Function2();
} catch(err) {
console.log(err);
}
try {
Function3();
} catch(err) {
console.log(err);
}
try {
Function4();
} catch(err) {
console.log(err);
}
I would like to deal with errors raised in different functions differently. It's possible that I catch all the errors in one catch block as illustrated in approach 1. But that way I have to make a big switch statement inside the catch block to differentiate different errors; moreover, if the errors thrown by different functions do not have a common switchable attribute I won't be able to use the switch statement at all; under such a situation, I have to use a separate try/catch block for each function call. Approach 2 sometimes is the only option. Does Promise not support this approach with its then/catch statement?
This can't be achieved with features of the language. However, pattern-based solutions are available.
Here are two solutions.
Rethrow previous error
This pattern is basically sound ...
Promise.resolve()
.then(Function1).catch(errorHandler1)
.then(Function2).catch(errorHandler2)
.then(Function3).catch(errorHandler3)
.then(Function4).catch(errorHandler4)
.catch(finalErrorHandler);
Promise.resolve() is not strictly necessary but allows all the .then().catch() lines to be of the same pattern, and the whole expression is easier on the eye.
... but :
if an errorHandler returns a result, then the chain will progress to the next line's success handler.
if an errorHandler throws, then the chain will progress to the next line's error handler.
The desired jump out of the chain won't happen unless the error handlers are written such that they can distinguish between a previously thrown error and a freshly thrown error. For example :
function errorHandler1(error) {
if (error instanceof MyCustomError) { // <<<<<<< test for previously thrown error
throw error;
} else {
// do errorHandler1 stuff then
// return a result or
// throw new MyCustomError() or
// throw new Error(), new RangeError() etc. or some other type of custom error.
}
}
Now :
if an errorHandler returns a result, then the chain will progress to the next FunctionN.
if an errorHandler throws a MyCustomError, then it will be repeatedly rethrown down the chain and caught by the first error handler that does not conform to the if(error instanceof MyCustomError) protocol (eg a final .catch()).
if an errorHandler throws any other type of error, then the chain will progress to the next catch.
This pattern would be useful if you need the flexibility to skip to end of chain or not, depending on the type of error thrown. Rare circumstances I expect.
DEMO
Insulated Catches
Another solution is to introduce a mechanism to keep each .catch(errorHandlerN) "insulated" such that it will catch only errors arising from its corresponding FunctionN, not from any preceding errors.
This can be achieved by having in the main chain only success handlers, each comprising an anonymous function containing a subchain.
Promise.resolve()
.then(function() { return Function1().catch(errorHandler1); })
.then(function() { return Function2().catch(errorHandler2); })
.then(function() { return Function3().catch(errorHandler3); })
.then(function() { return Function4().catch(errorHandler4); })
.catch(finalErrorHandler);
Here Promise.resolve() plays an important role. Without it, Function1().catch(errorHandler1) would be in the main chain the catch() would not be insulated from the main chain.
Now,
if an errorHandler returns a result, then the chain will progress to the next line.
if an errorHandler throws anything it likes, then the chain will progress directly to the finalErrorHandler.
Use this pattern if you want always to skip to the end of chain regardless of the type of error thrown. A custom error constructor is not required and the error handlers do not need to be written in a special way.
DEMO
Usage cases
Which pattern to choose will determined by the considerations already given but also possibly by the nature of your project team.
One-person team - you write everything and understand the issues - if you are free to choose, then run with your personal preference.
Multi-person team - one person writes the master chain and various others write the functions and their error handlers - if you can, opt for Insulated Catches - with everything under control of the master chain, you don't need to enforce the discipline of writing the error handlers in that certain way.
First off, I see a common mistake in this section of code that could be completely confusing you. This is your sample code block:
Promise.resolve(someFunction()).then(function() {
console.log('someFunction should throw error');
return -2;
}).catch(function(err) {
if (err instanceof MyCustomError) {
return -1;
}
}).then(someOtherFunction()); // <== Issue here
You need pass function references to a .then() handler, not actually call the function and pass their return result. So, this above code should probably be this:
Promise.resolve(someFunction()).then(function() {
console.log('someFunction should throw error');
return -2;
}).catch(function(err) {
if (err instanceof MyCustomError) {
// returning a normal value here will take care of the rejection
// and continue subsequent processing
return -1;
}
}).then(someOtherFunction); // just pass function reference here
Note that I've removed () after the functions in the .then() handler so you are just passing the function reference, not immediately calling the function. This will allow the promise infrastructure to decide whether to call the promise in the future or not. If you were making this mistake, it will totally throw you off for how the promises are working because things will get called regardless.
Three simple rules about catching rejections.
If nobody catches the rejection, it stops the promise chain immediately and the original rejection becomes the final state of the promise. No subsequent handlers are invoked.
If the promise rejection is caught and either nothing is returned or any normal value is returned from the reject handler, then the reject is considered handled and the promise chain continues and subsequent handlers are invoked. Whatever you return from the reject handler becomes the current value of the promise and it as if the reject never happened (except this level of resolve handler was not called - the reject handler was called instead).
If the promise reject is caught and you either throw an error from the reject handler or you return a rejected promise, then all resolve handlers are skipped until the next reject handler in the chain. If there are no reject handlers, then the promise chain is stopped and the newly minted error becomes the final state of the promise.
You can see a couple examples in this jsFiddle where it shows three situations:
Returning a regular value from a reject handler, causes the next .then() resolve handler to be called (e.g. normal processing continues),
Throwing in a reject handler causes normal resolve processing to stop and all resolve handlers are skipped until you get to a reject handler or the end of the chain. This is effective way to stop the chain if an unexpected error is found in a resolve handler (which I think is your question).
Not having a reject handler present causes normal resolve processing to stop and all resolve handlers are skipped until you get to a reject handler or the end of the chain.
There is no built-in functionality to skip the entirety of the remaining chain as you're requesting. However, you could imitate this behavior by throwing a certain error through each catch:
doSomething()
.then(func1).catch(handleError)
.then(func2).catch(handleError)
.then(func3).catch(handleError);
function handleError(reason) {
if (reason instanceof criticalError) {
throw reason;
}
console.info(reason);
}
If any of the catch blocks caught a criticalError they would skip straight to the end and throw the error. Any other error would be console logged and before continuing to the next .then block.
If you can use the newer async await this is pretty simple to implement:
async function myfunc() {
try {
return await anotherAsyncFunction();
} catch {
//do error handling
// can be async or not.
return errorObjct();
}
}
let alwaysGetAValue = await myfunc();
Depending on what technology your using you may need some kind of high level wrapper function to allow for the top level await.

Is an exception catch functionally the same as promise errorCallback?

I've been using javascript promsises thoughout my angular application but I am tired of repeating the same errorCallback for every promise. I am considering simply wrapping a promise in a try/catch block and having the catch block deal with any promise failures.
Which leads me to asking the question - are the error callbacks redundant if they are in a try/catch block? Will javascript 'catch' the error failure?
Edit: Adding code to reflect how I plan to do this:
try {
$http.post().success(callback);
}(catch)
{
}
The same as:
$http.post().succes(callback()).error(callback)
No, it won't. The reason is that what you are actually doing is queuing your callback function to be invoked when the request gets a successful, asynchronous response. By that time, the execution will have moved past your try/catch block, and you will get an unhandled error.
Another way to look at it is that synchronous, thrown errors immediately propagate up the call tree to the nearest catch block whereas asynchronous error results will only be handlable via the deferred object.
If you wish to invoke the same error handler for multiple promise-based asynchronous operations, you have multiple ways of going about it. The most obvious is injecting $q to your controller and doing something like this:
var deferred1 = $http.get(...);
var deferred2 = someOtherPromiseBasedMethod(...);
$q.all([deferred1, deferred2, ..., deferredN])
.then(null, function (rejection) {
// first failed deferred will provide the rejection value here
});
Another approach is to create a wrapper function:
function handleError(deferred) {
return deferred.then(null, commonErrorHandler);
}
handleError($http.get(...)).then(function (result) {
// Handle the success case here
}
That's still a bit repetitive, but less so, while being more of a declarative approach.

AngularJS - Promises rethrow caught exceptions

In the following code, an exception is caught by the catch function of the $q promise:
// Fiddle - http://jsfiddle.net/EFpn8/6/
f1().then(function(data) {
console.log("success 1: "+data)
return f2();
})
.then(function(data) {console.log("success 2: "+data)})
.catch(function(data) {console.log("error: "+data)});
function f1() {
var deferred = $q.defer();
// An exception thrown here is not caught in catch
// throw "err";
deferred.resolve("done f1");
return deferred.promise;
}
function f2() {
var deferred = $q.defer();
// An exception thrown here is handled properly
throw "err";
deferred.resolve("done f2");
return deferred.promise;
}
However when I look in the console log output I see the following:
The exception was caught in Angular, but was also caught by the error handling of the browser. This behavior does reproduce with Q library.
Is it a bug? How can I truly catch an exception with $q?
Angular's $q uses a convention where thrown errors are logged regardless of being caught. Instead, if you want to signal a rejection you need to return $q.reject(... as such:
function f2() {
var deferred = $q.defer();
// An exception thrown here is handled properly
return $q.reject(new Error("err"));//throw "err";
deferred.resolve("done f2");
return deferred.promise;
}
This is to distinguish rejections from errors like SyntaxError. Personally, it's a design choice I disagree with but it's understandable since $q is tiny so you can't really build in a reliable unhandled rejection detection mechanism. In stronger libraries like Bluebird, this sort of thing is not required.
As a side note - never, ever throw strings : you miss on stack traces that way.
Is it a bug?
No. Looking in the source for $q reveals that a deliberate try / catch block is created to respond to exceptions thrown in the callback by
Rejecting the promise, as through you had called deferred.reject
Calling the registered Angular exception hander. As can be seen in the $exceptionHandler docs, the default behaviour of this is to log it to the browser console as an error, which is what you have observed.
... was also caught by the error handling of the browser
To clarify, the exception isn't handled directly by the browser, but appears as an error because Angular has called console.error
How can I truly catch an exception with $q?
The callbacks are executed some time later, when the current call stack has cleared, so you won't be able to wrap the outer function in try / catch block. However, you have 2 options:
Put in try/catch block around the code that might throw the exception, within the callback:
f1().then(function(data) {
try {
return f2();
} catch(e) {
// Might want convert exception to rejected promise
return $q.reject(e);
}
})
Change how Angular's $exceptionHandler service behaves, like at How to override $exceptionHandler implementation . You could potentially change it to do absolutely nothing, so there would never be anything in the console's error log, but I don't think I would recommend that.
Fixed with AngularJS version 1.6
The reasoning for this behavior was that an uncaught error is different than a regular rejection, as
it can be caused by a programming error, for example. In practice, this turned out to be confusing
or undesirable for users, since neither native promises nor any other popular promise library
distinguishes thrown errors from regular rejections.
(Note: While this behavior does not go against the Promises/A+ spec, it is not prescribed either.)
$q:
Due to e13eea, an error thrown from a promise's onFulfilled or onRejection handlers is treated exactly the same as a regular rejection. Previously, it would also be passed to the $exceptionHandler() (in addition to rejecting the promise with the error as reason).
The new behavior applies to all services/controllers/filters etc that rely on $q (including built-in services, such as $http and $route). For example, $http's transformRequest/Response functions or a route's redirectTo function as well as functions specified in a route's resolve object, will no longer result in a call to $exceptionHandler() if they throw an error. Other than that, everything will continue to behave in the same way; i.e. the promises will be rejected, route transition will be cancelled, $routeChangeError events will be broadcasted etc.
-- AngularJS Developer Guide - Migrating from V1.5 to V1.6 - $q
The deferred is an outdated and a really terrible way of constructing promises, using the constructor solves this problem and more:
// This function is guaranteed to fulfill the promise contract
// of never throwing a synchronous exception, using deferreds manually
// this is virtually impossible to get right
function f1() {
return new Promise(function(resolve, reject) {
// code
});
}
I don't know if angular promises support the above, if not, you can do this:
function createPromise(fn) {
var d = $q.defer();
try {
fn(d.resolve.bind(d), d.reject.bind(d));
}
catch (e) {
d.reject(e);
}
return d.promise;
}
Usage is same as promise constructor:
function f1() {
return createPromise(function(resolve, reject){
// code
});
}
Here is an sample test that shows the new $q construction function, use of .finally(), rejections, and promise chain propagations:
iit('test',inject(function($q, $timeout){
var finallyCalled = false;
var failValue;
var promise1 = $q.when(true)
.then(function(){
return $q(function(resolve,reject){
// Reject promise1
reject("failed");
});
})
.finally(function(){
// Always called...
finallyCalled = true;
// This will be ignored
return $q.when('passed');
});
var promise2 = $q.when(promise1)
.catch(function(value){
// Catch reject of promise1
failValue = value;
// Continue propagation as resolved
return value+1;
// Or continue propagation as rejected
//return $q.reject(value+2);
});
var updateFailValue = function(val){ failValue = val; };
$q.when(promise2)
.then( updateFailValue )
.catch(updateFailValue );
$timeout.flush();
expect( finallyCalled ).toBe(true);
expect( failValue ).toBe('failed1');
}));

Using Kris Kowal's Q. How should I catch if any errors have been thrown throughout the life of a chained promise?

I'm new to promises, and I just started using Kris Kowal's Q today (I've read about it before and used it a little with Angular.js), and I was doing an Ajax call with jQuery and turning it into a Q promise. I was trying to find a good way to have some sort of catch at the end that would tell me whether this promise was rejected at one point or not so I could tell the user something went wrong along the way.
The idea is I want to extend the idea to work no matter how long my promise and at the end I want a check to see if it failed at anytime during the promise. This is what I ended up writing, but I wouldn't want to throw a new error in each of my fails/catches like this in order to propogate the error down the promise chain. I want something less weird.
function deleteEvent(id){
var url = "sampleURL/deleteEvent?id=" + id;
return Q($.ajax({
type: 'POST',
url: url,
dataType: 'json'
})).then(function (data) { // success
// on success
UpdateDOMforEventsChanged();
}).fail(function (xhr) { // failure
console.log(xhr);
throw new Error('Problem deleting Event');
});
}
And here's how I extended this promise elsewhere
deleteEvent(event._id).then(function () { // AJAX call was a success delete the things
$('#calendar').fullCalendar('removeEvents', event._id);
}).done(null, function () { // It failed somewhere alert the user.
alert("There was an error with deleting the event. Check your network connection and try again.")
});
So basically I've noticed that if I take out the error that I'm throwing in the fail section of my deleteEvent promise that it executes the following then that comes after it in the extended promise i.e. it executes deleting the events client side from fullCalendar. I don't want it to do this if it fails, how can I fix this? Also is there a reason why it does this? I'm guessing it might be, because you can resolve the error somehow or set a default and continue the promise chain...is that right?
I also notice if I don't throw the error it skips over the fail callback function of the done method. I know this is because the q documentation says the following about it's .done() method,
If there is an unhandled rejection, either because promise is
rejected and no onRejected callback was provided, or because
onFulfilled or onRejected threw an error or returned a rejected
promise, the resulting rejection reason is thrown as an exception in a
future turn of the event loop.
So I'm assuming I can't use .done() to catch all rejections including handled ones, (and I'm guessing in the ajax error is handled since I have the catch/fail after it), but what alternative is there?
A handled rejection is like a caught exception. It stops propagating since well, it was handled. If you want to handle the rejection and keep it rejected you need to rethrow, again, just like in synchronous code.
try {
throw new Error();
} catch(e){
// handle error
}
// no error here this code will keep running.
If you want it to keep rejecting and handle it, you need to rethrow:
try {
throw new Error();
} catch(e){
// handle error
throw e;
}
// this code will not run
The same with promises, just like you wrote. This is not particularly odd about promises, this is how synchronous exceptions work as well. If you want to propagate - you re-throw, otherwise - the errors are considered handled.
If you don't throw in fail it means you recover from error to success (see: Rejection into success [use cursors] mind fail is alias for catch), so result promise ends as successful. It can't be other way
Instead of using fail see if Q provides function which provides access to result and returns self promise (something like aside in implementation I mantain).
Also your flow can be simpler with less then's, see:
function deleteEvent(id) {
return Q($.ajax({
....
})).then(function (data) { // success
// on success
UpdateDOMforEventsChanged();
}); // No need for error handler
};
deleteEvent(event._id).done(function () { // AJAX call was a success delete the things
$('#calendar').fullCalendar('removeEvents', event._id);
}, , function (e) { // It failed somewhere alert the user.
console.error(e.stack || e.message);
alert("There was an error with deleting the event. Check your network connection and try again.")
});

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