I have error handling set up using try/catch blocks, which in its simplified form looks like this
try {
// .. Some application logic
try {
// .. some async api code
} catch (e) {
throw new Error("Api Error")
}
return true;
} catch (e) {
throw new Error("Unknown Error")
}
And the issue is, whenever I expect "Api Error" to be returned I get "Unknown Error" it seems like all errors are propagated to the outermost catch?
Is there a way to avoid this or another approach that allows for nested error handling?
In your code, check if exception is happening inside the first try block. If so, the inner try won't be executed.
try {
// .. Some application logic
// exception occurs here
// below code is not executed and the control is given to catch block
try {
//this did not execute
// .. some async api code
} catch (e) {
throw new Error("Api Error")
}
return true;
}
catch (e) {
//control came here
throw new Error("Unknown Error")
}
This is also a possible case:
try {
// .. Some application logic
// no exceptions occur here
// below code is executed
try {
//this did not execute
//exception here
// .. some async api code
} catch (e) {
//control came here and this threw APIerror
throw new Error("Api Error")
}
//this did not execute as the previous catch block raised another exception
return true;
}
catch (e) {
//control came here
throw new Error("Unknown Error")
}
Hope this helps :)
I'm writing a mocha clone, because mocha is lacking certain features that I need. But when I am using existing assertion libraries, I'm unable to catch errors that are returned by them. Here is the code that runs the tests.
Tester.Test = function(name, test_function) {
this.name = name;
this.run = function(callback) {
try {
test_function(callback);
} catch (e) {
callback(e);
}
};
};
I've already tried using domain to catch the error, but it's still not working:
var d = domain.create();
d.on("error", function(err) {
cb(err);
});
d.run(function() {
test.run(cb);
});
I still keep getting an AssertionError (expected false to be true). Any tips?
I have some code that I want to refactor (extract server communication methods from controller to separate service).
Example:
$http.post("/mypath", someData)
.success(function(request) {
if (request.ok) {
$scope.error = "";
_refreshAppointments();
}
else {
$scope.error = request.err;
}
})
.error(function() {
$scope.error = "Error during communicating to server";
});
My current problem is errors processing (communication with old $scope). So I want to throw the exceptions instead such lines $scope.error = "Error during communicating to server";
And catch them in controller.
Is it good idea?
If you throw an error in a vanilla environment:
setTimeout(function () {
throw new Error();
}, 1);
The error just gets lost. (window.onerror will see it though)
Even if you do:
try {
setTimeout(function () {
throw new Error();
}, 1);
} catch (e) {
console.log(e);
}
You wont see the error.
You need to wrap each asynchronous event, like:
function mySetTimeout(callback, ms) {
setTimeout(wrap_in_try_catch(callback), ms);
}
mySetTimeout(function () {
throw new Error();
});
You can now catch the error in a generic error handler, but you still can't catch it in the surrounding code.
This is where Promises come in. Not all libraries do Promises the same way (or correctly) so I don't know how good your library support is.
Roughly your code will look like:
$.ajax().then(function () {
throw new Error();
}).fail(e) {
console.log("it failed!", e);
});
If instead you have:
$.ajax().then(function () {
throw new Error();
}); // throws something like: no fail callback for rejected value error
Then your global error handler will pick it up. This ensures no error can slip through the cracks and get lost.
Getting a Promise library to work with Errors in this way is not impossible but it's a little bit tricky to set up. Once you have this though, you're good. Error handling becomes a breeze.
You'll never write a try-catch again, just a bunch of .fail() handlers.
It's definitely good idea to extract REST/http requests into model/service layer and use those services from controller. Then handling failed operation would mean rejecting a corresponding promise, in this case throwing exception in promise effectively means the same.
For example this is how your service/factory could look like:
app.factory('dataService', function($http) {
return {
load: function() {
return $http.post("/mypath", someData).then(function(response) {
if (!response.data.ok) {
return throw new Error(response.request.err);
// or return $q.reject(response.request.err);
}
return response.request;
});
}
};
});
and consuming controller would deal with promise status, resolved (success) or rejected (failed/exception):
dataService.load().then(function(request) {
$scope.error = "";
_refreshAppointments();
})
.catch(function(err) {
$scope.error = err || "Error during communicating to server";
});
How can I stop a thrown error from propagating all the way down the chain? It shows in my catch() block but it doesn't stop and crashes the server with an uncaught exception.
I am running this as part of a node cron job (node-cron) as:
var cronJob = require('cron').CronJob;
var cron = require('../lib/cron')
var c = new cronJob('* * * * * *', function() {
console.log('Cron starting');
mycode.run();
}, function() {
console.log('Cron executed');
}, true);
c.start();
In my cron.js
module.exports = {
run: function() {
return job.getAndStore().catch(function(e) {
// This prints but it keeps on going so to speak - it doesn't 'catch', just notifies me
console.log('ERROR', e);
});
}
};
Console dump:
Cron starting
ERROR [TypeError: undefined is not a function]
Cron starting
Uncaught Exception
[TypeError: undefined is not a function]
TypeError: undefined is not a function
I have to do this which I know not quite right:
try {
run();
} catch(e) {
console.log('Now it stops')
}
The run() is part of some cron library that doesn't have any promise support so I am wrapping it in the function to call it.
Edit As I think my issue is related to subsequent calls I believe it has to do with how I handle the Mongo connection on 2+ calls:
// Create a Mongo connection
Job.prototype.getDb = function(id) {
var self = this;
return new P(function(resolve, reject) {
if (!self.db) {
return Mongo.connectAsync(self.options.connection)
.then(function(c) {
self.db = c;
debug('Got new connection');
resolve(c);
});
}
debug('Got existing connection');
resolve(self.db);
});
};
// Fetch stuff
Job.prototype.getAndStore = function(c) {
return this.getDb().then(function() {
throw new Error('Boom');
});
};
Your catch callback is only executed the first time. You are getting the uncaught exception in the second run of the cron job, and it looks like your job.getAndStore() does not return a rejected promise there but throws synchronously. It shouldn't, it should always return a promise.
You can use Bluebirds Promise.try to automatically catch such exceptions and transform them into a promise rejection. Or you wrap your getAndStore function in Promise.method:
var safeGetAndStore = Promise.method(job.getAndStore.bind(job));
module.exports = {
run: function() {
return safeGetAndStore().catch(function(e) {
console.log('ERROR', e);
});
}
};
In your specific case, the problem was that your job did cache the db connection and returned that when it was already available - but you needed to return a promise with a .then method. You should simply cache the promise itself:
Job.prototype.getDb = function(id) {
if (!this.db) {
this.db = Mongo.connectAsync(self.options.connection);
return this.db;
};
Use done, at least if bluebird implements it properly it will work as you expect.
catch(..) is just alias for then(null, ..) which is promise transformer that creates another promise for further processing.
So following should work for you:
module.exports = {
run: function() {
return job.getAndStore().done(null, function(e) {
// This prints but it keeps on going so to speak - it doesn't 'catch', just notifies me
console.log('ERROR', e);
});
}
};
I am still fairly new to promises and am using bluebird currently, however I have a scenario where I am not quite sure how to best deal with it.
So for example I have a promise chain within an express app like so:
repository.Query(getAccountByIdQuery)
.catch(function(error){
res.status(404).send({ error: "No account found with this Id" });
})
.then(convertDocumentToModel)
.then(verifyOldPassword)
.catch(function(error) {
res.status(406).send({ OldPassword: error });
})
.then(changePassword)
.then(function(){
res.status(200).send();
})
.catch(function(error){
console.log(error);
res.status(500).send({ error: "Unable to change password" });
});
So the behaviour I am after is:
Goes to get account by Id
If there is a rejection at this point, bomb out and return an error
If there is no error convert the document returned to a model
Verify the password with the database document
If the passwords dont match then bomb out and return a different error
If there is no error change the passwords
Then return success
If anything else went wrong, return a 500
So currently catches do not seem to stop the chaining, and that makes sense, so I am wondering if there is a way for me to somehow force the chain to stop at a certain point based upon the errors, or if there is a better way to structure this to get some form of branching behaviour, as there is a case of if X do Y else Z.
Any help would be great.
This behavior is exactly like a synchronous throw:
try{
throw new Error();
} catch(e){
// handle
}
// this code will run, since you recovered from the error!
That's half of the point of .catch - to be able to recover from errors. It might be desirable to rethrow to signal the state is still an error:
try{
throw new Error();
} catch(e){
// handle
throw e; // or a wrapper over e so we know it wasn't handled
}
// this code will not run
However, this alone won't work in your case since the error be caught by a later handler. The real issue here is that generalized "HANDLE ANYTHING" error handlers are a bad practice in general and are extremely frowned upon in other programming languages and ecosystems. For this reason Bluebird offers typed and predicate catches.
The added advantage is that your business logic does not (and shouldn't) have to be aware of the request/response cycle at all. It is not the query's responsibility to decide which HTTP status and error the client gets and later as your app grows you might want to separate the business logic (how to query your DB and how to process your data) from what you send to the client (what http status code, what text and what response).
Here is how I'd write your code.
First, I'd get .Query to throw a NoSuchAccountError, I'd subclass it from Promise.OperationalError which Bluebird already provides. If you're unsure how to subclass an error let me know.
I'd additionally subclass it for AuthenticationError and then do something like:
function changePassword(queryDataEtc){
return repository.Query(getAccountByIdQuery)
.then(convertDocumentToModel)
.then(verifyOldPassword)
.then(changePassword);
}
As you can see - it's very clean and you can read the text like an instruction manual of what happens in the process. It is also separated from the request/response.
Now, I'd call it from the route handler as such:
changePassword(params)
.catch(NoSuchAccountError, function(e){
res.status(404).send({ error: "No account found with this Id" });
}).catch(AuthenticationError, function(e){
res.status(406).send({ OldPassword: error });
}).error(function(e){ // catches any remaining operational errors
res.status(500).send({ error: "Unable to change password" });
}).catch(function(e){
res.status(500).send({ error: "Unknown internal server error" });
});
This way, the logic is all in one place and the decision of how to handle errors to the client is all in one place and they don't clutter eachother.
.catch works like the try-catch statement, which means you only need one catch at the end:
repository.Query(getAccountByIdQuery)
.then(convertDocumentToModel)
.then(verifyOldPassword)
.then(changePassword)
.then(function(){
res.status(200).send();
})
.catch(function(error) {
if (/*see if error is not found error*/) {
res.status(404).send({ error: "No account found with this Id" });
} else if (/*see if error is verification error*/) {
res.status(406).send({ OldPassword: error });
} else {
console.log(error);
res.status(500).send({ error: "Unable to change password" });
}
});
I am wondering if there is a way for me to somehow force the chain to stop at a certain point based upon the errors
No. You cannot really "end" a chain, unless you throw an exception that bubbles until its end. See Benjamin Gruenbaum's answer for how to do that.
A derivation of his pattern would be not to distinguish error types, but use errors that have statusCode and body fields which can be sent from a single, generic .catch handler. Depending on your application structure, his solution might be cleaner though.
or if there is a better way to structure this to get some form of branching behaviour
Yes, you can do branching with promises. However, this means to leave the chain and "go back" to nesting - just like you'd do in an nested if-else or try-catch statement:
repository.Query(getAccountByIdQuery)
.then(function(account) {
return convertDocumentToModel(account)
.then(verifyOldPassword)
.then(function(verification) {
return changePassword(verification)
.then(function() {
res.status(200).send();
})
}, function(verificationError) {
res.status(406).send({ OldPassword: error });
})
}, function(accountError){
res.status(404).send({ error: "No account found with this Id" });
})
.catch(function(error){
console.log(error);
res.status(500).send({ error: "Unable to change password" });
});
I have been doing this way:
You leave your catch in the end. And just throw an error when it happens midway your chain.
repository.Query(getAccountByIdQuery)
.then((resultOfQuery) => convertDocumentToModel(resultOfQuery)) //inside convertDocumentToModel() you check for empty and then throw new Error('no_account')
.then((model) => verifyOldPassword(model)) //inside convertDocumentToModel() you check for empty and then throw new Error('no_account')
.then(changePassword)
.then(function(){
res.status(200).send();
})
.catch((error) => {
if (error.name === 'no_account'){
res.status(404).send({ error: "No account found with this Id" });
} else if (error.name === 'wrong_old_password'){
res.status(406).send({ OldPassword: error });
} else {
res.status(500).send({ error: "Unable to change password" });
}
});
Your other functions would probably look something like this:
function convertDocumentToModel(resultOfQuery) {
if (!resultOfQuery){
throw new Error('no_account');
} else {
return new Promise(function(resolve) {
//do stuff then resolve
resolve(model);
}
}
Probably a little late to the party, but it is possible to nest .catch as shown here:
Mozilla Developer Network - Using Promises
Edit: I submitted this because it provides the asked functionality in general. However it doesn't in this particular case. Because as explained in detail by others already, .catch is supposed to recover the error. You can't, for example, send a response to the client in multiple .catch callbacks because a .catch with no explicit return resolves it with undefined in that case, causing proceeding .then to trigger even though your chain is not really resolved, potentially causing a following .catch to trigger and sending another response to the client, causing an error and likely throwing an UnhandledPromiseRejection your way. I hope this convoluted sentence made some sense to you.
Instead of .then().catch()... you can do .then(resolveFunc, rejectFunc). This promise chain would be better if you handled things along the way. Here is how I would rewrite it:
repository.Query(getAccountByIdQuery)
.then(
convertDocumentToModel,
() => {
res.status(404).send({ error: "No account found with this Id" });
return Promise.reject(null)
}
)
.then(
verifyOldPassword,
() => Promise.reject(null)
)
.then(
changePassword,
(error) => {
if (error != null) {
res.status(406).send({ OldPassword: error });
}
return Promise.Promise.reject(null);
}
)
.then(
_ => res.status(200).send(),
error => {
if (error != null) {
console.error(error);
res.status(500).send({ error: "Unable to change password" });
}
}
);
Note: The if (error != null) is a bit of a hack to interact with the most recent error.
I think Benjamin Gruenbaum's answer above is the best solution for a complex logic sequence, but here is my alternative for simpler situations. I just use an errorEncountered flag along with return Promise.reject() to skip any subsequent then or catch statements. So it would look like this:
let errorEncountered = false;
someCall({
/* do stuff */
})
.catch({
/* handle error from someCall*/
errorEncountered = true;
return Promise.reject();
})
.then({
/* do other stuff */
/* this is skipped if the preceding catch was triggered, due to Promise.reject */
})
.catch({
if (errorEncountered) {
return;
}
/* handle error from preceding then, if it was executed */
/* if the preceding catch was executed, this is skipped due to the errorEncountered flag */
});
If you have more than two then/catch pairs, you should probably use Benjamin Gruenbaum's solution. But this works for a simple set-up.
Note that the final catch only has return; rather than return Promise.reject();, because there's no subsequent then that we need to skip, and it would count as an unhandled Promise rejection, which Node doesn't like. As is written above, the final catch will return a peacefully resolved Promise.
I wanted to preserve the branching behaviour that Bergi's answer had, yet still provide the clean code structure of unnested .then()'s
If you can handle some ugliness in the machinery that makes this code work, the result is a clean code structure similar to non-nested chained .then()'s
One nice part of structuring a chain like this, is that you can handle all the potential results in one place by chainRequests(...).then(handleAllPotentialResults) this might be nice if you need to hide the request chain behind some standardised interface.
const log = console.log;
const chainRequest = (stepFunction, step) => (response) => {
if (response.status === 200) {
return stepFunction(response, step);
}
else {
log(`Failure at step: ${step}`);
return response;
}
};
const chainRequests = (initialRequest, ...steps) => {
const recurs = (step) => (response) => {
const incStep = step + 1;
const nextStep = steps.shift();
return nextStep ? nextStep(response, step).then(chainRequest(recurs(incStep), incStep)) : response;
};
return initialRequest().then(recurs(0));
};
// Usage
async function workingExample() {
return await chainRequests(
() => fetch('https://jsonplaceholder.typicode.com/users'),
(resp, step) => { log(`step: ${step}`, resp); return fetch('https://jsonplaceholder.typicode.com/posts/'); },
(resp, step) => { log(`step: ${step}`, resp); return fetch('https://jsonplaceholder.typicode.com/posts/3'); }
);
}
async function failureExample() {
return await chainRequests(
() => fetch('https://jsonplaceholder.typicode.com/users'),
(resp, step) => { log(`step: ${step}`, resp); return fetch('https://jsonplaceholder.typicode.com/posts/fail'); },
(resp, step) => { log(`step: ${step}`, resp); return fetch('https://jsonplaceholder.typicode.com/posts/3'); }
);
}
console.log(await workingExample());
console.log(await failureExample());
The idea is there, but the interface exposed could probably use some tweaking.
Seeing as this implementation used curried arrow functions, the above could potentially be implemented with more direct async/await code