I'm wanting to clarify how a promise is passed to .catch and what .catch does with it.
Using this as an example:
function fetchDog(){
fetch("https://dog.ceo/api/breeds/image/fail")
.then(response => response.json())
.then(data => console.log(data))
.catch(function(err) {
console.log('Fetch problem');
});
};
fetchDog();
Looking at this statement from MDN:
If the Promise that then is called on adopts a state (fulfillment or
rejection) for which then has no handler, a new Promise is created
with no additional handlers, simply adopting the final state of the
original Promise on which then was called.
I translate that to mean, in my example, the .thens return a new promise that is a copy of the promise that .then was called on.
By the time it reaches .catch, I know that .catch prints something to the console. The spec also says it behaves the same as calling Promise.prototype.then(undefined, onRejected).
Therefore, based on this excerpt from the .then spec:
If a handler function: doesn't return anything, the promise returned
by then gets resolved with an undefined value.
I expect .catch to return a new promise that 'gets resolved with' an undefined value. (What exactly does that mean, for a promise object to get 'resolved with' an undefined value)?
Is this true?
.then() calls its callback when the promise is resolved. .catch() only calls its callback when the promise is rejected.
If fetch() is successful, it resolves its promise, so only the .then() callbacks are called.
If fetch() gets an error, it rejects its promise, and the .catch() callbacks will be called. Also, if response.json() gets an error (e.g. the response was not valid JSON), it will reject the promise, and .catch() will call its callback.
Related
I have trouble understanding the difference between putting .catch BEFORE and AFTER then in a nested promise.
Alternative 1:
test1Async(10).then((res) => {
return test2Async(22)
.then((res) => {
return test3Async(100);
}).catch((err) => {
throw "ERROR AFTER THEN";
});
}).then((res) => {
console.log(res);
}).catch((err) => {
console.log(err);
});
Alternative 2:
test1Async(10).then((res) => {
return test2Async(22)
.catch((err) => {
throw "ERROR BEFORE THEN";
})
.then((res) => {
return test3Async(100);
});
}).then((res) => {
console.log(res);
}).catch((err) => {
console.log(err);
});
The behavior of each function is as follow, test1 fail if number is <0 test2 fails if number is > 10 and test3 fails if number is not 100. In this case test2 is only failing.
I tried to run and make test2Async fail, both BEFORE and AFTER then behaves the same way and that is not executing the test3Async. Can somebody explain to me the main difference for placing catch in different places?
In each function I console.log('Running test X') in order to check if it gets executed.
This question arises because of the previous thread I posted How to turn nested callback into promise?. I figure it is a different problem and worth posting another topic.
So, basically you're asking what is the difference between these two (where p is a promise created from some previous code):
return p.then(...).catch(...);
and
return p.catch(...).then(...);
There are differences either when p resolves or rejects, but whether those differences matter or not depends upon what the code inside the .then() or .catch() handlers does.
What happens when p resolves:
In the first scheme, when p resolves, the .then() handler is called. If that .then() handler either returns a value or another promise that eventually resolves, then the .catch() handler is skipped. But, if the .then() handler either throws or returns a promise that eventually rejects, then the .catch() handler will execute for both a reject in the original promise p, but also an error that occurs in the .then() handler.
In the second scheme, when p resolves, the .then() handler is called. If that .then() handler either throws or returns a promise that eventually rejects, then the .catch() handler cannot catch that because it is before it in the chain.
So, that's difference #1. If the .catch() handler is AFTER, then it can also catch errors inside the .then() handler.
What happens when p rejects:
Now, in the first scheme, if the promise p rejects, then the .then() handler is skipped and the .catch() handler will be called as you would expect. What you do in the .catch() handler determines what is returned as the final result. If you just return a value from the .catch() handler or return a promise that eventually resolves, then the promise chain switches to the resolved state because you "handled" the error and returned normally. If you throw or return a rejected promise in the .catch() handler, then the returned promise stays rejected.
In the second scheme, if the promise p rejects, then the .catch() handler is called. If you return a normal value or a promise that eventually resolves from the .catch() handler (thus "handling" the error), then the promise chain switches to the resolved state and the .then() handler after the .catch() will be called.
So that's difference #2. If the .catch() handler is BEFORE, then it can handle the error and allow the .then() handler to still get called.
When to use which:
Use the first scheme if you want just one .catch() handler that can catch errors in either the original promise p or in the .then() handler and a reject from p should skip the .then() handler.
Use the second scheme if you want to be able to catch errors in the original promise p and maybe (depending upon conditions), allow the promise chain to continue as resolved, thus executing the .then() handler.
The other option
There's one other option to use both callbacks that you can pass to .then() as in:
p.then(fn1, fn2)
This guarantees that only one of fn1 or fn2 will ever be called. If p resolves, then fn1 will be called. If p rejects, then fn2 will be called. No change of outcome in fn1 can ever make fn2 get called or vice versa. So, if you want to make absolutely sure that only one of your two handlers is called regardless of what happens in the handlers themselves then you can use p.then(fn1, fn2).
jfriend00's answer is excellent, but I thought it would be a good idea to add the analogous synchronous code.
return p.then(...).catch(...);
is similar to the synchronous:
try {
iMightThrow() // like `p`
then()
} catch (err) {
handleCatch()
}
If iMightThrow() doesn't throw, then() will be called. If it does throw (or if then() itself throws), then handleCatch() will be called. Notice how the catch block has no control over whether or not then is called.
On the other hand,
return p.catch(...).then(...);
is similar to the synchronous:
try {
iMightThrow()
} catch (err) {
handleCatch()
}
then()
In this case, if iMightThrow() doesn't throw, then then() will execute. If it does throw, then it would be up to handleCatch() to decide if then() is called, because if handleCatch() rethrows, then then() will not be called, since the exception will be thrown to the caller immediately. If handleCatch() can gracefully handle the issue, then then() will be called.
for the following promise chain
new Promise(r=>r('b').then(()=>Promise.reject('x'))).catch(err=>console.log({error}))
the inner promise is returned, so it should bubble up and catched by the outer catch block and logs the error to the console.
but instead, it got resolved.
Promise {<fulfilled>: 'b'}
how to catch the inner promise by an outer catch block?
r (more typically called resolve) does not return a promise, so calling .then on it is an error. Normally, if an error is thrown in a promise executor function (the function you pass new Promise), the promise being created is rejected with that error. But since you've already called resolve, you've fulfilled the promise with "b", and once fulfilled it cannot be changed to being rejected, so the error occurring during the executor is just suppressed. (This is a specific case of the more general rule that once a promise is resolved [tied to an outcome] it can't be resolved differently. A promise can be resolved without [yet] being fulfilled, but fulfilling it is one way of resolving it. If you're not 100% on this promise terminology, check out my blog post here explaining it.)
If you want to resolve the promise you're creating to another promise, you pass it into the resolve function:
new Promise(resolve => {
resolve(Promise.reject(new Error("x")));
})
.then(value => console.log("value", value))
.catch(error => console.error("error", error.message));
That specific example would be an example of the Explicit promise construction antipattern (we should just use the promise from Promise.reject directly), but if you had branching logic in the promise executor and some of the branches didn't involve promises, you might do something similar to this (though stylistically it would make more sense to me to throw an error or call the reject function passed to the executor).
function fetchDog(){
fetch("https://dog.ceo/api/breeds/image/fail")
.then(response => response.json())
.then(data => console.log(data))
.catch(function(err) {
console.log('Fetch problem');
});
};
fetchDog();
Using the above example I would like to clarify how .catch here receives the rejected Promise. It's also a good exercise in reading MDN for me.
1 .
Looking at this statement from MDN:
If the Promise that then is called on adopts a state (fulfillment or
rejection) for which then has no handler, a new Promise is created
with no additional handlers, simply adopting the final state of the
original Promise on which then was called.
I translate that to mean, in my example, the .thens return a new promise that is a copy of the promise that .then was called on.
2 .
The Promise.prototype.catch spec also says it behaves the same as calling Promise.prototype.then(undefined, onRejected).
I interpret this to mean, in my example, that the first callback in catch is the onRejected parameter. Therefore, when catch receives a rejected promise, it executes console.log('Fetch problem');.
I also interpret this to mean that when catch invariably receives a fulfilled promise, it returns undefined? (I haven't thought of a way to test this in the console).
3 .
I also read in the .then spec:
If a handler function: doesn't return anything, the promise returned
by then gets resolved with an undefined value.
Therefore, in my code snippet, I interpret this to mean catch returns a promise whose value is undefined.
Based on this understanding, so long as the fetch line returned a fulfilled promise, this fulfilled promise would find its way to catch and catch's callback wouldn't execute. catch would return undefined. (I can't think of a way to test this). I suspect my understanding is wrong.
When catch invariably receives a fulfilled promise, it returns undefined?
No. Calling .catch() will always return a promise. It does that before even knowing whether the promise that it was called on is fulfilled, rejected or still pending.
I interpret this to mean catch returns a promise whose value is undefined.
Yes. You can test this easily with
function handleError(p1) {
const p2 = p1.catch(err => {
console.log('handling problem', err);
});
p2.then(res => {
console.log('final promise fulfilled with', res);
});
}
// handleError(Promise.resolve('success'));
handleError(Promise.reject('error'));
So I am wondering if this works?
S3.getObject()
.promise()
.then()
.catch() // catch error from the first then() statement
.then()
.catch() // catch error from the second then() statement
or do I need to place all 'catches' in the end? Can I have multiple catch then? Will they be fired in the order of the 'then' statements throwing errors?
It depends of your actual goals.
As a matter of fact, .then() method takes two parameters:
onFullfilled: Callback to be invoked when the promise is fulfilled.
onRejected: Callback to be invoked when the promise is rejected.
In fact, .catch(fn) is just a shorthand for .then(null, fn).
Both .then() and .catch() each return a new promise which resolves to its return value. In other words:
A resolved promise of that value if it isn't a promise.
The actual return value if it is already a promise (that will be fulfilled or rejected).
A rejected promise if the return value is a rejected promise (as previous point says) or any error is thrown.
The main reason behind the use of .then(onFullfill).catch(onReject) pattern instead of .then(onFullfill, onReject) is that, in the former (which is equivalent to .then(onFullfill).then(null, onReject)), we are chaining the onReject callback to the promise returned by first .then() instead of directly to the original promise.
The consequence of this is that if en error is thrown inside the onFullfill callback (or it returns a promise which happen to resolve to a rejected state), it will be catched by the chained .catch() too.
So, answering to your question, when you do something like:
P.then(...)
.then(...)
.then(...)
.catch(...)
;
You are chaining promises "supposing" all will go fine "and only check at the end". That is: Whenever any step fails, all subsequent .then()s are bypassed up to the next (in this case the last) .catch().
On the other hand, if you insert more .catch()s in between, you would be able to intercept rejected promises earlier and, if appropriate, solve whatever were going on and turn it into a resolved state again in order to resume the chain.
I have trouble understanding the difference between putting .catch BEFORE and AFTER then in a nested promise.
Alternative 1:
test1Async(10).then((res) => {
return test2Async(22)
.then((res) => {
return test3Async(100);
}).catch((err) => {
throw "ERROR AFTER THEN";
});
}).then((res) => {
console.log(res);
}).catch((err) => {
console.log(err);
});
Alternative 2:
test1Async(10).then((res) => {
return test2Async(22)
.catch((err) => {
throw "ERROR BEFORE THEN";
})
.then((res) => {
return test3Async(100);
});
}).then((res) => {
console.log(res);
}).catch((err) => {
console.log(err);
});
The behavior of each function is as follow, test1 fail if number is <0 test2 fails if number is > 10 and test3 fails if number is not 100. In this case test2 is only failing.
I tried to run and make test2Async fail, both BEFORE and AFTER then behaves the same way and that is not executing the test3Async. Can somebody explain to me the main difference for placing catch in different places?
In each function I console.log('Running test X') in order to check if it gets executed.
This question arises because of the previous thread I posted How to turn nested callback into promise?. I figure it is a different problem and worth posting another topic.
So, basically you're asking what is the difference between these two (where p is a promise created from some previous code):
return p.then(...).catch(...);
and
return p.catch(...).then(...);
There are differences either when p resolves or rejects, but whether those differences matter or not depends upon what the code inside the .then() or .catch() handlers does.
What happens when p resolves:
In the first scheme, when p resolves, the .then() handler is called. If that .then() handler either returns a value or another promise that eventually resolves, then the .catch() handler is skipped. But, if the .then() handler either throws or returns a promise that eventually rejects, then the .catch() handler will execute for both a reject in the original promise p, but also an error that occurs in the .then() handler.
In the second scheme, when p resolves, the .then() handler is called. If that .then() handler either throws or returns a promise that eventually rejects, then the .catch() handler cannot catch that because it is before it in the chain.
So, that's difference #1. If the .catch() handler is AFTER, then it can also catch errors inside the .then() handler.
What happens when p rejects:
Now, in the first scheme, if the promise p rejects, then the .then() handler is skipped and the .catch() handler will be called as you would expect. What you do in the .catch() handler determines what is returned as the final result. If you just return a value from the .catch() handler or return a promise that eventually resolves, then the promise chain switches to the resolved state because you "handled" the error and returned normally. If you throw or return a rejected promise in the .catch() handler, then the returned promise stays rejected.
In the second scheme, if the promise p rejects, then the .catch() handler is called. If you return a normal value or a promise that eventually resolves from the .catch() handler (thus "handling" the error), then the promise chain switches to the resolved state and the .then() handler after the .catch() will be called.
So that's difference #2. If the .catch() handler is BEFORE, then it can handle the error and allow the .then() handler to still get called.
When to use which:
Use the first scheme if you want just one .catch() handler that can catch errors in either the original promise p or in the .then() handler and a reject from p should skip the .then() handler.
Use the second scheme if you want to be able to catch errors in the original promise p and maybe (depending upon conditions), allow the promise chain to continue as resolved, thus executing the .then() handler.
The other option
There's one other option to use both callbacks that you can pass to .then() as in:
p.then(fn1, fn2)
This guarantees that only one of fn1 or fn2 will ever be called. If p resolves, then fn1 will be called. If p rejects, then fn2 will be called. No change of outcome in fn1 can ever make fn2 get called or vice versa. So, if you want to make absolutely sure that only one of your two handlers is called regardless of what happens in the handlers themselves then you can use p.then(fn1, fn2).
jfriend00's answer is excellent, but I thought it would be a good idea to add the analogous synchronous code.
return p.then(...).catch(...);
is similar to the synchronous:
try {
iMightThrow() // like `p`
then()
} catch (err) {
handleCatch()
}
If iMightThrow() doesn't throw, then() will be called. If it does throw (or if then() itself throws), then handleCatch() will be called. Notice how the catch block has no control over whether or not then is called.
On the other hand,
return p.catch(...).then(...);
is similar to the synchronous:
try {
iMightThrow()
} catch (err) {
handleCatch()
}
then()
In this case, if iMightThrow() doesn't throw, then then() will execute. If it does throw, then it would be up to handleCatch() to decide if then() is called, because if handleCatch() rethrows, then then() will not be called, since the exception will be thrown to the caller immediately. If handleCatch() can gracefully handle the issue, then then() will be called.