I had a look at the bluebird promise FAQ, in which it mentions that .then(success, fail) is an antipattern. I don't quite understand its explanation as for the try and catch.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
It seems that the example is suggesting the following to be the correct way.
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
What's the difference?
What's the difference?
The .then() call will return a promise that will be rejected in case the callback throws an error. This means, when your success logger fails, the error would be passed to the following .catch() callback, but not to the fail callback that goes alongside success.
Here's a control flow diagram:
To express it in synchronous code:
// some_promise_call().then(logger.log, logger.log)
then: {
try {
var results = some_call();
} catch(e) {
logger.log(e);
break then;
} // else
logger.log(results);
}
The second log (which is like the first argument to .then()) will only be executed in the case that no exception happened. The labelled block and the break statement feel a bit odd, this is actually what python has try-except-else for (recommended reading!).
// some_promise_call().then(logger.log).catch(logger.log)
try {
var results = some_call();
logger.log(results);
} catch(e) {
logger.log(e);
}
The catch logger will also handle exceptions from the success logger call.
So much for the difference.
I don't quite understand its explanation as for the try and catch
The argument is that usually, you want to catch errors in every step of the processing and that you shouldn't use it in chains. The expectation is that you only have one final handler which handles all errors - while, when you use the "antipattern", errors in some of the then-callbacks are not handled.
However, this pattern is actually very useful: When you want to handle errors that happened in exactly this step, and you want to do something entirely different when no error happened - i.e. when the error is unrecoverable. Be aware that this is branching your control flow. Of course, this is sometimes desired.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
That you had to repeat your callback. You rather want
some_promise_call()
.catch(function(e) {
return e; // it's OK, we'll just log it
})
.done(function(res) {
logger.log(res);
});
You also might consider using .finally() for this.
The two aren't quite identical. The difference is that the first example won't catch an exception that's thrown in your success handler. So if your method should only ever return resolved promises, as is often the case, you need a trailing catch handler (or yet another then with an empty success parameter). Sure, it may be that your then handler doesn't do anything that might potentially fail, in which case using one 2-parameter then could be fine.
But I believe the point of the text you linked to is that then is mostly useful versus callbacks in its ability to chain a bunch of asynchronous steps, and when you actually do this, the 2-parameter form of then subtly doesn't behave quite as expected, for the above reason. It's particularly counterintuitive when used mid-chain.
As someone who's done a lot of complex async stuff and bumped into corners like this more than I care to admit, I really recommend avoiding this anti-pattern and going with the separate handler approach.
By looking at advantages and disadvantages of both we can make a calculated guess as to which is appropriate for the situation.
These are the two main approaches to implementing promises. Both have it's pluses and minus
Catch Approach
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
Advantages
All errors are handled by one catch block.
Even catches any exception in the then block.
Chaining of multiple success callbacks
Disadvantages
In case of chaining it becomes difficult to show different error messages.
Success/Error Approach
some_promise_call()
.then(function success(res) { logger.log(res) },
function error(err) { logger.log(err) })
Advantages
You get fine grained error control.
You can have common error handling function for various categories of errors like db error, 500 error etc.
Disavantages
You will still need another catch if you wish to handler errors thrown by the success callback
Simple explain:
In ES2018
When the catch method is called with argument onRejected, the
following steps are taken:
Let promise be the this value.
Return ? Invoke(promise, "then", « undefined, onRejected »).
that means:
promise.then(f1).catch(f2)
equals
promise.then(f1).then(undefiend, f2)
Using .then().catch() lets you enable Promise Chaining which is required to fulfil a workflow. You may need to read some information from database then you want to pass it to an async API then you want to manipulate the response. You may want to push the response back into the database. Handling all these workflows with your concept is doable but very hard to manage. The better solution will be then().then().then().then().catch() which receives all errors in just once catch and lets you keep the maintainability of the code.
Using then() and catch() helps chain success and failure handler on the promise.catch() works on promise returned by then(). It handles,
If promise was rejected. See #3 in the picture
If error occurred in success handler of then(), between line numbers 4 to 7 below. See #2.a in the picture
(Failure callback on then() does not handle this.)
If error occurred in failure handler of then(), line number 8 below. See #3.b in the picture.
1. let promiseRef: Promise = this. aTimetakingTask (false);
2. promiseRef
3. .then(
4. (result) => {
5. /* successfully, resolved promise.
6. Work on data here */
7. },
8. (error) => console.log(error)
9. )
10. .catch( (e) => {
11. /* successfully, resolved promise.
12. Work on data here */
13. });
Note: Many times, failure handler might not be defined if catch() is
written already.
EDIT: reject() result in invoking catch() only if the error
handler in then() is not defined. Notice #3 in the picture to
the catch(). It is invoked when handler in line# 8 and 9 are not
defined.
It makes sense because promise returned by then() does not have an error if a callback is taking care of it.
Instead of words, good example. Following code (if first promise resolved):
Promise.resolve()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
is identical to:
Promise.resolve()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
But with rejected first promise, this is not identical:
Promise.reject()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
Promise.reject()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
Related
I had a look at the bluebird promise FAQ, in which it mentions that .then(success, fail) is an antipattern. I don't quite understand its explanation as for the try and catch.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
It seems that the example is suggesting the following to be the correct way.
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
What's the difference?
What's the difference?
The .then() call will return a promise that will be rejected in case the callback throws an error. This means, when your success logger fails, the error would be passed to the following .catch() callback, but not to the fail callback that goes alongside success.
Here's a control flow diagram:
To express it in synchronous code:
// some_promise_call().then(logger.log, logger.log)
then: {
try {
var results = some_call();
} catch(e) {
logger.log(e);
break then;
} // else
logger.log(results);
}
The second log (which is like the first argument to .then()) will only be executed in the case that no exception happened. The labelled block and the break statement feel a bit odd, this is actually what python has try-except-else for (recommended reading!).
// some_promise_call().then(logger.log).catch(logger.log)
try {
var results = some_call();
logger.log(results);
} catch(e) {
logger.log(e);
}
The catch logger will also handle exceptions from the success logger call.
So much for the difference.
I don't quite understand its explanation as for the try and catch
The argument is that usually, you want to catch errors in every step of the processing and that you shouldn't use it in chains. The expectation is that you only have one final handler which handles all errors - while, when you use the "antipattern", errors in some of the then-callbacks are not handled.
However, this pattern is actually very useful: When you want to handle errors that happened in exactly this step, and you want to do something entirely different when no error happened - i.e. when the error is unrecoverable. Be aware that this is branching your control flow. Of course, this is sometimes desired.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
That you had to repeat your callback. You rather want
some_promise_call()
.catch(function(e) {
return e; // it's OK, we'll just log it
})
.done(function(res) {
logger.log(res);
});
You also might consider using .finally() for this.
The two aren't quite identical. The difference is that the first example won't catch an exception that's thrown in your success handler. So if your method should only ever return resolved promises, as is often the case, you need a trailing catch handler (or yet another then with an empty success parameter). Sure, it may be that your then handler doesn't do anything that might potentially fail, in which case using one 2-parameter then could be fine.
But I believe the point of the text you linked to is that then is mostly useful versus callbacks in its ability to chain a bunch of asynchronous steps, and when you actually do this, the 2-parameter form of then subtly doesn't behave quite as expected, for the above reason. It's particularly counterintuitive when used mid-chain.
As someone who's done a lot of complex async stuff and bumped into corners like this more than I care to admit, I really recommend avoiding this anti-pattern and going with the separate handler approach.
By looking at advantages and disadvantages of both we can make a calculated guess as to which is appropriate for the situation.
These are the two main approaches to implementing promises. Both have it's pluses and minus
Catch Approach
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
Advantages
All errors are handled by one catch block.
Even catches any exception in the then block.
Chaining of multiple success callbacks
Disadvantages
In case of chaining it becomes difficult to show different error messages.
Success/Error Approach
some_promise_call()
.then(function success(res) { logger.log(res) },
function error(err) { logger.log(err) })
Advantages
You get fine grained error control.
You can have common error handling function for various categories of errors like db error, 500 error etc.
Disavantages
You will still need another catch if you wish to handler errors thrown by the success callback
Simple explain:
In ES2018
When the catch method is called with argument onRejected, the
following steps are taken:
Let promise be the this value.
Return ? Invoke(promise, "then", « undefined, onRejected »).
that means:
promise.then(f1).catch(f2)
equals
promise.then(f1).then(undefiend, f2)
Using .then().catch() lets you enable Promise Chaining which is required to fulfil a workflow. You may need to read some information from database then you want to pass it to an async API then you want to manipulate the response. You may want to push the response back into the database. Handling all these workflows with your concept is doable but very hard to manage. The better solution will be then().then().then().then().catch() which receives all errors in just once catch and lets you keep the maintainability of the code.
Using then() and catch() helps chain success and failure handler on the promise.catch() works on promise returned by then(). It handles,
If promise was rejected. See #3 in the picture
If error occurred in success handler of then(), between line numbers 4 to 7 below. See #2.a in the picture
(Failure callback on then() does not handle this.)
If error occurred in failure handler of then(), line number 8 below. See #3.b in the picture.
1. let promiseRef: Promise = this. aTimetakingTask (false);
2. promiseRef
3. .then(
4. (result) => {
5. /* successfully, resolved promise.
6. Work on data here */
7. },
8. (error) => console.log(error)
9. )
10. .catch( (e) => {
11. /* successfully, resolved promise.
12. Work on data here */
13. });
Note: Many times, failure handler might not be defined if catch() is
written already.
EDIT: reject() result in invoking catch() only if the error
handler in then() is not defined. Notice #3 in the picture to
the catch(). It is invoked when handler in line# 8 and 9 are not
defined.
It makes sense because promise returned by then() does not have an error if a callback is taking care of it.
Instead of words, good example. Following code (if first promise resolved):
Promise.resolve()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
is identical to:
Promise.resolve()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
But with rejected first promise, this is not identical:
Promise.reject()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
Promise.reject()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
In learning to write in JavaScript with Promises, I'm encountering two different ways of dealing with errors. One is to use a catch, as in this example:
axios.post('/someapi', {
somedata1: 'foo'
})
.then((result) => {
console.log(result);
})
.catch((exception) => {
console.log(exception);
});
The other is to have a clause in the then for the rejected case:
axios.post('/someapi', {
somedata1: 'foo',
})
.then((response) => {
console.log(response);
}, (error) => {
console.log(error);
});
Some authors seem to use one approach, other authors use the other, and it's not clear to me why. Are there situations in which it's necessary or desirable to use one or the other?
(The example above uses axios, but that's just for the purposes of providing a code example. This question is not about axios.)
With Javascript Promises, are there best practices regarding the use of “error” versus catch clauses?
There is no universal "best practice" for that question because it depends upon what specific behavior you want your code to have. As others have mentioned, you get a different behavior in a few ways.
Some authors seem to use one approach, other authors use the other, and it's not clear to me why. Are there situations in which it's necessary or desirable to use one or the other?
Yes, there are situations to use one or the other. They provide potentially different behaviors. Only if you're 200% sure that your successHandler in .then(successHandler) can never throw or return a rejected promise, would there be no meaningful difference.
As a summary:
When using p.then(successHandler).catch(errorHandler), errorHandler will get errors that occur either from a rejected p or from an error or rejection from the successHandler.
When using p.then(successHandler, errorHandler), errorHandler will be called from a rejection of p and will NOT get called from an error or rejection from the successHandler.
Different behaviors that are useful for different circumstances.
In this .catch() example below, the .catch() will catch an error that occurs (either accidentally from a coding error, a thrown exception or when returning some other promise that rejects).
Promise.resolve("hello").then(greeting => {
console.log("throwing error");
throw new Error("My .then() handler had an error");
}).catch(err => {
// will get here
console.log("Caught error in .catch()\nError was: ", err.message);
});
But, when using the second argument to .then(), that error from the .then() handler will not be caught:
Promise.resolve("hello").then(greeting => {
console.log("throwing error");
throw new Error("My .then() handler had an error");
}, err => {
// won't get here
console.log("Caught error in .catch()\nError was: ", err.message);
});
So, sometimes you want an error that might occur in the .then() handler to hit this immediate error handler and sometimes you don't want it to hit that error handler because you want that error handler to only process errors from the original promise and you have some other catch handler later in the promise chain that will deal with this error.
Recommendation
In general, I would advise that you start out with the .catch() handler because it catches more errors and does not require that there be some other .catch() handler elsewhere in the promise chain in order to be safe. Then, you switch to the .then(successHandler, errorHandler) form if you explicitly don't want this errorHandlerto be called if there's another error in the successHandler AND you have somewhere else in the promise chain where an error in the successHandler would get caught or handled.
Both the syntaxes work out of the box. But the first one has an advantage that the Catch block is able to catch an error thrown by the Promise Rejection as well as an error thrown by then block.
Here is the Example you can try in Node and you will have a better idea about it.
function x () {
return new Promise((resolve, reject) => {
return resolve('Done...');
});
}
x()
.then(response => {
console.log('RESPONSE---', response)
throw 'Oops...Error occurred...';
})
.catch(error => console.log('ERROR---', error));
x()
.then(
response => {
console.log('RESPONSE---', response)
throw 'Oops...Error occurred...';
},
error => console.log('ERROR---', error)
);
I had a look at the bluebird promise FAQ, in which it mentions that .then(success, fail) is an antipattern. I don't quite understand its explanation as for the try and catch.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
It seems that the example is suggesting the following to be the correct way.
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
What's the difference?
What's the difference?
The .then() call will return a promise that will be rejected in case the callback throws an error. This means, when your success logger fails, the error would be passed to the following .catch() callback, but not to the fail callback that goes alongside success.
Here's a control flow diagram:
To express it in synchronous code:
// some_promise_call().then(logger.log, logger.log)
then: {
try {
var results = some_call();
} catch(e) {
logger.log(e);
break then;
} // else
logger.log(results);
}
The second log (which is like the first argument to .then()) will only be executed in the case that no exception happened. The labelled block and the break statement feel a bit odd, this is actually what python has try-except-else for (recommended reading!).
// some_promise_call().then(logger.log).catch(logger.log)
try {
var results = some_call();
logger.log(results);
} catch(e) {
logger.log(e);
}
The catch logger will also handle exceptions from the success logger call.
So much for the difference.
I don't quite understand its explanation as for the try and catch
The argument is that usually, you want to catch errors in every step of the processing and that you shouldn't use it in chains. The expectation is that you only have one final handler which handles all errors - while, when you use the "antipattern", errors in some of the then-callbacks are not handled.
However, this pattern is actually very useful: When you want to handle errors that happened in exactly this step, and you want to do something entirely different when no error happened - i.e. when the error is unrecoverable. Be aware that this is branching your control flow. Of course, this is sometimes desired.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
That you had to repeat your callback. You rather want
some_promise_call()
.catch(function(e) {
return e; // it's OK, we'll just log it
})
.done(function(res) {
logger.log(res);
});
You also might consider using .finally() for this.
The two aren't quite identical. The difference is that the first example won't catch an exception that's thrown in your success handler. So if your method should only ever return resolved promises, as is often the case, you need a trailing catch handler (or yet another then with an empty success parameter). Sure, it may be that your then handler doesn't do anything that might potentially fail, in which case using one 2-parameter then could be fine.
But I believe the point of the text you linked to is that then is mostly useful versus callbacks in its ability to chain a bunch of asynchronous steps, and when you actually do this, the 2-parameter form of then subtly doesn't behave quite as expected, for the above reason. It's particularly counterintuitive when used mid-chain.
As someone who's done a lot of complex async stuff and bumped into corners like this more than I care to admit, I really recommend avoiding this anti-pattern and going with the separate handler approach.
By looking at advantages and disadvantages of both we can make a calculated guess as to which is appropriate for the situation.
These are the two main approaches to implementing promises. Both have it's pluses and minus
Catch Approach
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
Advantages
All errors are handled by one catch block.
Even catches any exception in the then block.
Chaining of multiple success callbacks
Disadvantages
In case of chaining it becomes difficult to show different error messages.
Success/Error Approach
some_promise_call()
.then(function success(res) { logger.log(res) },
function error(err) { logger.log(err) })
Advantages
You get fine grained error control.
You can have common error handling function for various categories of errors like db error, 500 error etc.
Disavantages
You will still need another catch if you wish to handler errors thrown by the success callback
Simple explain:
In ES2018
When the catch method is called with argument onRejected, the
following steps are taken:
Let promise be the this value.
Return ? Invoke(promise, "then", « undefined, onRejected »).
that means:
promise.then(f1).catch(f2)
equals
promise.then(f1).then(undefiend, f2)
Using .then().catch() lets you enable Promise Chaining which is required to fulfil a workflow. You may need to read some information from database then you want to pass it to an async API then you want to manipulate the response. You may want to push the response back into the database. Handling all these workflows with your concept is doable but very hard to manage. The better solution will be then().then().then().then().catch() which receives all errors in just once catch and lets you keep the maintainability of the code.
Using then() and catch() helps chain success and failure handler on the promise.catch() works on promise returned by then(). It handles,
If promise was rejected. See #3 in the picture
If error occurred in success handler of then(), between line numbers 4 to 7 below. See #2.a in the picture
(Failure callback on then() does not handle this.)
If error occurred in failure handler of then(), line number 8 below. See #3.b in the picture.
1. let promiseRef: Promise = this. aTimetakingTask (false);
2. promiseRef
3. .then(
4. (result) => {
5. /* successfully, resolved promise.
6. Work on data here */
7. },
8. (error) => console.log(error)
9. )
10. .catch( (e) => {
11. /* successfully, resolved promise.
12. Work on data here */
13. });
Note: Many times, failure handler might not be defined if catch() is
written already.
EDIT: reject() result in invoking catch() only if the error
handler in then() is not defined. Notice #3 in the picture to
the catch(). It is invoked when handler in line# 8 and 9 are not
defined.
It makes sense because promise returned by then() does not have an error if a callback is taking care of it.
Instead of words, good example. Following code (if first promise resolved):
Promise.resolve()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
is identical to:
Promise.resolve()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
But with rejected first promise, this is not identical:
Promise.reject()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
Promise.reject()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
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.
I had a look at the bluebird promise FAQ, in which it mentions that .then(success, fail) is an antipattern. I don't quite understand its explanation as for the try and catch.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
It seems that the example is suggesting the following to be the correct way.
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
What's the difference?
What's the difference?
The .then() call will return a promise that will be rejected in case the callback throws an error. This means, when your success logger fails, the error would be passed to the following .catch() callback, but not to the fail callback that goes alongside success.
Here's a control flow diagram:
To express it in synchronous code:
// some_promise_call().then(logger.log, logger.log)
then: {
try {
var results = some_call();
} catch(e) {
logger.log(e);
break then;
} // else
logger.log(results);
}
The second log (which is like the first argument to .then()) will only be executed in the case that no exception happened. The labelled block and the break statement feel a bit odd, this is actually what python has try-except-else for (recommended reading!).
// some_promise_call().then(logger.log).catch(logger.log)
try {
var results = some_call();
logger.log(results);
} catch(e) {
logger.log(e);
}
The catch logger will also handle exceptions from the success logger call.
So much for the difference.
I don't quite understand its explanation as for the try and catch
The argument is that usually, you want to catch errors in every step of the processing and that you shouldn't use it in chains. The expectation is that you only have one final handler which handles all errors - while, when you use the "antipattern", errors in some of the then-callbacks are not handled.
However, this pattern is actually very useful: When you want to handle errors that happened in exactly this step, and you want to do something entirely different when no error happened - i.e. when the error is unrecoverable. Be aware that this is branching your control flow. Of course, this is sometimes desired.
What's wrong with the following?
some_promise_call()
.then(function(res) { logger.log(res) }, function(err) { logger.log(err) })
That you had to repeat your callback. You rather want
some_promise_call()
.catch(function(e) {
return e; // it's OK, we'll just log it
})
.done(function(res) {
logger.log(res);
});
You also might consider using .finally() for this.
The two aren't quite identical. The difference is that the first example won't catch an exception that's thrown in your success handler. So if your method should only ever return resolved promises, as is often the case, you need a trailing catch handler (or yet another then with an empty success parameter). Sure, it may be that your then handler doesn't do anything that might potentially fail, in which case using one 2-parameter then could be fine.
But I believe the point of the text you linked to is that then is mostly useful versus callbacks in its ability to chain a bunch of asynchronous steps, and when you actually do this, the 2-parameter form of then subtly doesn't behave quite as expected, for the above reason. It's particularly counterintuitive when used mid-chain.
As someone who's done a lot of complex async stuff and bumped into corners like this more than I care to admit, I really recommend avoiding this anti-pattern and going with the separate handler approach.
By looking at advantages and disadvantages of both we can make a calculated guess as to which is appropriate for the situation.
These are the two main approaches to implementing promises. Both have it's pluses and minus
Catch Approach
some_promise_call()
.then(function(res) { logger.log(res) })
.catch(function(err) { logger.log(err) })
Advantages
All errors are handled by one catch block.
Even catches any exception in the then block.
Chaining of multiple success callbacks
Disadvantages
In case of chaining it becomes difficult to show different error messages.
Success/Error Approach
some_promise_call()
.then(function success(res) { logger.log(res) },
function error(err) { logger.log(err) })
Advantages
You get fine grained error control.
You can have common error handling function for various categories of errors like db error, 500 error etc.
Disavantages
You will still need another catch if you wish to handler errors thrown by the success callback
Simple explain:
In ES2018
When the catch method is called with argument onRejected, the
following steps are taken:
Let promise be the this value.
Return ? Invoke(promise, "then", « undefined, onRejected »).
that means:
promise.then(f1).catch(f2)
equals
promise.then(f1).then(undefiend, f2)
Using .then().catch() lets you enable Promise Chaining which is required to fulfil a workflow. You may need to read some information from database then you want to pass it to an async API then you want to manipulate the response. You may want to push the response back into the database. Handling all these workflows with your concept is doable but very hard to manage. The better solution will be then().then().then().then().catch() which receives all errors in just once catch and lets you keep the maintainability of the code.
Using then() and catch() helps chain success and failure handler on the promise.catch() works on promise returned by then(). It handles,
If promise was rejected. See #3 in the picture
If error occurred in success handler of then(), between line numbers 4 to 7 below. See #2.a in the picture
(Failure callback on then() does not handle this.)
If error occurred in failure handler of then(), line number 8 below. See #3.b in the picture.
1. let promiseRef: Promise = this. aTimetakingTask (false);
2. promiseRef
3. .then(
4. (result) => {
5. /* successfully, resolved promise.
6. Work on data here */
7. },
8. (error) => console.log(error)
9. )
10. .catch( (e) => {
11. /* successfully, resolved promise.
12. Work on data here */
13. });
Note: Many times, failure handler might not be defined if catch() is
written already.
EDIT: reject() result in invoking catch() only if the error
handler in then() is not defined. Notice #3 in the picture to
the catch(). It is invoked when handler in line# 8 and 9 are not
defined.
It makes sense because promise returned by then() does not have an error if a callback is taking care of it.
Instead of words, good example. Following code (if first promise resolved):
Promise.resolve()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
is identical to:
Promise.resolve()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)
But with rejected first promise, this is not identical:
Promise.reject()
.then
(
() => { throw new Error('Error occurs'); },
err => console.log('This error is caught:', err)
);
Promise.reject()
.catch
(
err => console.log('This error is caught:', err)
)
.then
(
() => { throw new Error('Error occurs'); }
)