Develop Reference

Promise chain continues after rejection

I'm having trouble to properly catch an error/reject in a promise chain.
const p1 = () => {
return new Promise((resolve, reject) => {
console.log("P1");
resolve();
});
};
const p2 = () => {
return new Promise((resolve, reject) => {
console.log("P2");
reject();
});
};
const p3 = () => {
return new Promise((resolve, reject) => {
console.log("P3");
resolve();
});
};
p1().catch(() => {
console.log("Caught p1");
}).then(p2).catch(() => {
console.log("Caught p2");
}).then(p3).catch(() => {
console.log("Caught p3");
}).then(() => {
console.log("Final then");
});
When the promise is rejected, the following .then still gets executed. In my understanding, when in a promise chain an error/reject happened, the .then calls that follow it are not executed any more.
P1
P2
Caught p2
P3
Final then
The rejection gets caught correctly, but why is "P3" logged after the catch?
What am I doing wrong?
To clarify #evolutionxbox, this is my expected result:
Promise.resolve().then(() => {
console.log("resolve #1");
return Promise.reject();
}).then(() => {
console.log("resolve #2");
return Promise.resolve();
}).then(() => {
console.log("resolve #3");
return Promise.resolve();
}).then(() => {
console.log("Final end");
}).catch(() => {
console.log("Caught");
});
This code works exactly like it should. And I can't see a difference to my code, except that I declared the functions separately.
The code above stops no matter where the promise is rejected.

Here is a synchronous equivalent of your code:
const f1 = () => {
console.log("F1");
};
const f2 = () => {
console.log("F2");
throw new Error();
};
const f3 = () => {
console.log("F3");
};
try {
f1();
} catch {
console.log("Caught f1");
}
try {
f2();
} catch {
console.log("Caught f2");
}
try {
f3();
} catch {
console.log("Caught f3");
}
console.log("Final code");
As you can see, that gives a matching result. Hopefully, looking at the synchronous code you would not be surprised why. In a try..catch you are allowed to attempt recovery. The idea is that the catch will stop the error propagation and you can hopefully continue further. Or if you do want to stop, you still have to explicitly throw again, for example:
doCode();
try {
makeCoffee();
} catch(err) {
if (err instanceof IAmATeapotError) {
//attempt recovery
makeTea();
} else {
//unrecoverable - log and re-throw
console.error("Fatal coffee related issue encountered", err);
throw err;
}
}
doCode();
This is also the purpose Promise#catch() serves - so you can attempt recovery or at least act when there was a problem. The idea is that after the .catch() you might be able to continue:
const orderPizza = (topping) =>
new Promise((resolve, reject) => {
if (topping === "pepperoni")
reject(new Error("No pepperoni available"));
else
resolve(`${topping} pizza`);
});
const makeToast = () => "toast";
const eat = food => console.log(`eating some ${food}`);
async function main() {
await orderPizza("cheese")
.catch(makeToast)
.then(eat);
console.log("-----");
await orderPizza("pepperoni")
.catch(makeToast)
.then(eat);
}
main();
In order to reject the promise chain from a .catch() you need to do something similar as a normal catch and fail at the error recovery by inducing another error. You can throw or return a rejected promise to that effect.
This code works exactly like it should. And I can't see a difference to my code, except that I declared the functions separately.
The code above stops no matter where the promise is rejected.
The second piece of code you show fails entirely after a reject because there are no other .catch()-es that are successful. It is basically similar to this synchronous code:
try {
console.log("log #1");
throw new Error();
console.log("log #2");
console.log("log #3");
console.log("Final end");
} catch {
console.log("Caught");
}
Thus if you do not want to recover early, you can also skip the .catch() instead of inducing another error.

Try this.
const p1 = (arg) => {
// Promise returns data in the respected arguments
return new Promise((resolve, reject) => {
// Data to be accessed through first argument.
resolve(arg);
});
};
const p2 = (arg) => {
return new Promise((resolve, reject) => {
// Data to be accessed through second argument.
reject(arg);
});
}
p1('p1').then(resolve => {
console.log(resolve + ' is handled with the resolve argument. So it is accessed with .then()');
}) // Since reject isn't configured to pass any data we don't use .catch()
p2('p2').catch(reject => {
console.log(reject + ' is handled with the reject argument. So it is accessed with .catch()');
}) // Since resolve ins't configured to pass any data we don't use .then()
// You would normally configure a Promise to return a value on with resolve, and access it with .then() when it completes a task successfully.
// .catch() would then be chained on to the end of .then() to handle errors when a task cannot be completed.
// Here is an example.
const p3 = () => {
return new Promise((resolve, reject) => {
var condition = true;
if (condition === true) {
resolve('P3');
} else {
reject('Promise failed!');
}
});
};
p3('p3').then(resolve => {
console.log(resolve);
}).catch(reject => {
console.log(reject);
})

You do not do anything wrong.
In your code you call the first promise p1. Then you write p1.catch(...).then(...).then(...).then(...). This is a chain which means that you should call then 3 times, because you called resolve method in the p1 promise (all these thens depend on the first promise).

When the promise is rejected, the following .then still gets executed.
Yes. Just to be accurate: the then and catch method calls are all executed synchronously (in one go), and so all promises involved are created in one go. It's the callbacks passed to these methods that execute asynchronously, as the relevant promises resolve (fullfill or reject).
In my understanding, when in a promise chain an error/reject happened, the .then calls that follow it are not executed any more.
This is not the case. The promise that a catch returns can either fullfill or reject depending on what happens in the callback passed to it, and so the callbacks further down the chain will execute accordingly when that promise resolves.
The rejection gets caught correctly, but why is "P3" logged after the catch?
As in your case the catch callback returns undefined (it only performs a console.log), its promise fullfulls! By consequence, the chained then callback -- on that promise -- is executed... etc.
If you want to "stop"
If you want to keep the chain as it is, but wish to have a behaviour where a rejection leads to no further execution of then or catch callbacks, then don't resolve the associated promise:
const stop = new Promise(resolve => null);
const p1 = () => {
return new Promise((resolve, reject) => {
console.log("P1");
resolve();
});
};
const p2 = () => {
return new Promise((resolve, reject) => {
console.log("P2");
reject();
});
};
const p3 = () => {
return new Promise((resolve, reject) => {
console.log("P3");
resolve();
});
};
p1().catch(() => {
console.log("Caught p1");
return stop; // don't resolve
}).then(p2).catch(() => {
console.log("Caught p2");
return stop;
}).then(p3).catch(() => {
console.log("Caught p3");
return stop;
}).then(() => {
console.log("Final then");
});

Am I chaining Promises correctly or committing a sin?

I have not worked with Javascript in a long time, so now promises are a new concept to me. I have some operations requiring more than one asynchronous call but which I want to treat as a transaction where steps do not execute if the step before failed. Currently I chain promises by nesting and I want to return a promise to the caller.
After reading the chaining section of Mozilla's Using Promises guide, I'm not sure if what I'm doing is correct or equivalent to the "callback pyramid of doom".
Is there a cleaner way to do this (besides chaining with a guard check in each then)? Am I right in my belief that in Mozilla's example it will execute each chained then even when there is an error?
myfunction(key) => {
return new Promise((outerResolve, outerReject) => {
return new Promise((resolve, reject) => {
let item = cache.get(key);
if (item) {
resolve(item);
} else {
//we didnt have the row cached, load it from store
chrome.storage.sync.get(key, function (result) {
chrome.runtime.lastError
? reject({ error: chrome.runtime.lastError.message })
: resolve(result);
});
}
}).then((resolve) => {
//Now the inner most item is resolved, we are working in the 'outer' shell
if (resolve.error) {
outerReject(resolve);
} else {
//No error, continue
new Promise((resolve, reject) => {
chrome.storage.sync.get(keyBasedOnPreviousData, function (result) {
chrome.runtime.lastError
? reject({ error: chrome.runtime.lastError.message })
: resolve(result);
});
}).then((resolve) => {
//finally return the result to the caller
if (resolve.error) {
outerReject(resolve);
} else {
outerResolve(resolve);
}
});
}
});
});
}

Subsequent then statements are not executed (until a catch) when an exception is thrown. Also, .then returns a Promise, so you don't need to create an additional, outer Promise.
Try this example:
var p = new Promise((resolve, reject) => {
console.log('first promise, resolves');
resolve();
})
.then(() => {
throw new Error('Something failed');
})
.then(() => {
console.log('then after the error');
return('result');
});
p.then(res => console.log('success: ' + res), err => console.log('error: ' + err));
You will not see "then after the error" in the console, because that happens after an exception is thrown. But if you comment the throw statement, you will get the result you expect in the Promise.
I am not sure I understand your example entirely, but I think it could be simplified like this:
myfunction(key) => {
return new Promise((resolve, reject) => {
let item = cache.get(key);
if (item) {
resolve(item);
} else {
//we didnt have the row cached, load it from store
chrome.storage.sync.get(key, function (result) {
chrome.runtime.lastError
? throw new Error(chrome.runtime.lastError.message)
: resolve(result);
});
}
}).then((previousData) => {
// keyBasedOnPreviousData is calculated based on previousData
chrome.storage.sync.get(keyBasedOnPreviousData, function (result) {
chrome.runtime.lastError
? throw new Error(chrome.runtime.lastError.message)
: return result;
});
});
}

It's a bit of a mess. This is my attempt at rewriting. A good thing to try to avoid is new Promise().
function chromeStorageGet(key) {
return new Promise( (res, rej) => {
chrome.storage.sync.get(key, result => {
if (chrome.runtime.lastError) {
rej(new Error(chrome.runtime.lastError.message))
} else {
res(result)
}
});
});
});
function myfunction(key) {
const item = cache.get(key) ? Promise.resolve(cache.get(key)) : chromeStorageGet(key);
return item.then( cacheResult => {
return chromeStorageGet(keyBasedOnPreviousData);
});
}
Why avoid new Promise()?
The reason for this is that you want to do every step with then(). If any error happened in any of the promises, every promise in the chain will fail and any subsequent then() will not get executed until there is a catch() handler.
Lots of promise based-code requires no error handlers, because promise-based functions always return promises and exceptions should flow all the back to the caller until there is something useful to be done with error handling.
Note that the exceptions to these 2 rules are in my chromeStorageGet function. A few notes here:
new Promise can be a quick and easy way to convert callback code to promise code.
It's usually a good idea to just create a little conversion layer for this callback-based code. If you need chrome.storage.sync in other places, maybe create a little utility that promisifies all its functions.
If there is only 1 'flow', you can just use a series of then() to complete the process, but sometimes you need to conditionally do other things. Just splitting up these complicated operations in a number of different functions can really help here.
But this:
const result = condition ? Promise.resolve() : Promise.reject();
Is almost always preferred to:
const result = new Promise( (res, rej) => {
if (condition) {
res();
} else {
rej();
}
}

Promise chain with an asynchronous operation not executing in order

I have found other people asking about this topic but I haven't been able to get my promise chain to execute in order.
Here is a basic reproduction of what is happening:
function firstMethod(){
dbHelper.executeQuery(queryParameters).then(result => {
if (result === whatIAmExpecting) {
return dbHelper.doDbOperation(secondQueryParameters)}
else {
throw new Error('An error occurred')
}})
.then(doFinalOperation())
.catch(error => {
})
}
In the above code doFinalOperation() is called before the then function after executeQuery() is called.
Here is the implementation of executeQuery():
function executeQuery(parameter) {
return new Promise((resolve, reject) => {
const queryToExecute = `SELECT * FROM parameter`
return mySqlConnection.query(queryToExecute).then((result) => {
resolve(result)
}).catch(error => {
reject(error)
})
})
And here is the implementation of of the mySqlConnection.query method:
function query(queryString){
return new Promise((resolve, reject) =>
{
initConnection()
connection.connect()
require('bluebird').promisifyAll(connection)
return connection.queryAsync(queryString).then(function(results) {
connection.end();
resolve(results)
}).catch(error => {
reject(error)
})
})
It seems like I have incorrectly implemented the executeQuery() method. The database operation in the mySqlConnection.query is asychronous and I can see that's where the chain of promises stops happening in the expected order.
My question in a nutshell: How do I make the my chain of promises execute in order, and how I do stop a then() method from being executed before the previous Promise has called resolve() or reject()?
Thanks in advance.

then expects a function, but you have accidentally executed it, instead of passing it. Change:
then(doFinalOperation())
with:
then(doFinalOperation)
Now it will be the promise implementation that invokes it (at the proper time), not "you".
If your function needs arguments to be passed, then you can either
(1) Use bind:
then(doFinalOperation.bind(null, parameterOne, parameterTwo, parameterThree))
(2) Use a function expression
then(_ => doFinalOperation(parameterOne, parameterTwo, parameterThree))

Both your .then() methods get called on the first async operation...
Should be something like this:
function firstMethod(){
dbHelper.executeQuery(queryParameters).then(expectedResult => {
if (expectedResult === whatIAmExpecting) {
return dbHelper.doDbOperation(secondQueryParameters)}
.then(doFinalOperation())
.catch(error => {
};
}
else {
throw new Error('An error occurred')
}})
.catch(error => {
});
}

Promise.all().then() goes to catch() because then is not recognized

The code is running well until the Promise.all and then is goes right to the catch saying 'then is not defined'.
I've been trying to figure this out without success for hours :(.
Any help are welcome.
Here is a simplified example of code:
// Save
return new Promise((fulfillSave, rejectSave) => {
// Get
this._getObjects().then((object) => {
var promises = [];
// For each value
object.values.forEach((value) => {
promises.push(
// Create promise
new Promise((fulfill, reject) => {
// Create MDB object + assign value
valueMongoDB.value = value;
// Save
valueMongoDB.save((err, results) => {
if (err) {
reject('Error in saving');
} else {
fulfill();
}
});
})
);
});
// Wait for all promises
Promise.all(promises).then(() => {
// Nothing to do
fulfillSave();
}, then((err) => {
// Err
rejectSave(err);
}));
}
}).catch((err) => {
rejectSave(`Error: ${err.message}`);
});
});
Thanks in advance!
Serge.

This is incorrect:
// Wait for all promises
Promise.all(promises).then(() => {
// Nothing to do
fulfillSave();
}, then((err) => {
// ^^^^--------------------------- error here
// Err
rejectSave(err);
}));
It's trying to call a freestanding function called then and pass its return value into the then on the object returned by Promise.all.
I think you're trying to hook up a failure handler. If so, you don't say then, you just supply a second function:
Promise.all(promises).then(() => {
// Nothing to do
fulfillSave();
}, (err) => {
// Err
rejectSave(err);
}));
But of course, since you're not using the result of that chain and you're just passing the single argument your second function receives into rejectSave, you could just pass rejectSave directly:
Promise.all(promises).then(() => {
// Nothing to do
fulfillSave();
}, rejectSave);
If you told us what your overall code is meant to do and what its inputs are, my suspicion is that that code could be a lot simpler. It's common to create and nest promises unnecessarily, and I suspect that's happening here.
For instance, if you just want to do the saves and get back a promise that will resolve when they're all done successfully or reject on the first failure:
return this._getObjects()
.then(objects => Promise.all(objects.map(value => {
return new Promise((resolve, reject) => {
// Create MDB object + assign value
valueMongoDB.value = value;
// Save
valueMongoDB.save((err, results) => {
if (err) {
reject('Error in saving');
} else {
fulfill();
}
});
});
})));
Or if we give ourselves a helper function for the Mongo bit:
function mongoSavePromise(value) {
return new Promise((resolve, reject) => {
// Create MDB object + assign value
valueMongoDB.value = value;
// Save
valueMongoDB.save((err, results) => {
if (err) {
reject('Error in saving');
} else {
fulfill();
}
});
});
}
then:
return this._getObjects()
.then(objects => Promise.all(objects.map(mongoSavePromise)));

Avoid the Promise constructor antipattern!
Your whole code should be a simple
return this._getObjects().then(object => {
var promises = object.values.map(value => {
// Create MDB object + assign value
valueMongoDB.value = value;
// Save
return valueMongoDB.save().catch(err => {
throw 'Error in saving';
});
});
// Wait for all promises
return Promise.all(promises);
}, err => {
throw `Error: ${err.message}`;
});
No unnecessary callbacks, no room for mistakes. Btw, you shouldn't throw strings.

JavaScript Promises - reject vs. throw

I have read several articles on this subject, but it is still not clear to me if there is a difference between Promise.reject vs. throwing an error. For example,
Using Promise.reject
return asyncIsPermitted()
.then(function(result) {
if (result === true) {
return true;
}
else {
return Promise.reject(new PermissionDenied());
}
});
Using throw
return asyncIsPermitted()
.then(function(result) {
if (result === true) {
return true;
}
else {
throw new PermissionDenied();
}
});
My preference is to use throw simply because it is shorter, but was wondering if there is any advantage of one over the other.

There is no advantage of using one vs the other, but, there is a specific case where throw won't work. However, those cases can be fixed.
Any time you are inside of a promise callback, you can use throw. However, if you're in any other asynchronous callback, you must use reject.
For example, this won't trigger the catch:
new Promise(function() {
setTimeout(function() {
throw 'or nah';
// return Promise.reject('or nah'); also won't work
}, 1000);
}).catch(function(e) {
console.log(e); // doesn't happen
});
Instead you're left with an unresolved promise and an uncaught exception. That is a case where you would want to instead use reject. However, you could fix this in two ways.
by using the original Promise's reject function inside the timeout:
new Promise(function(resolve, reject) {
setTimeout(function() {
reject('or nah');
}, 1000);
}).catch(function(e) {
console.log(e); // works!
});
by promisifying the timeout:
function timeout(duration) { // Thanks joews
return new Promise(function(resolve) {
setTimeout(resolve, duration);
});
}
timeout(1000).then(function() {
throw 'worky!';
// return Promise.reject('worky'); also works
}).catch(function(e) {
console.log(e); // 'worky!'
});

Another important fact is that reject() DOES NOT terminate control flow like a return statement does. In contrast throw does terminate control flow.
Example:
new Promise((resolve, reject) => {
throw "err";
console.log("NEVER REACHED");
})
.then(() => console.log("RESOLVED"))
.catch(() => console.log("REJECTED"));
vs
new Promise((resolve, reject) => {
reject(); // resolve() behaves similarly
console.log("ALWAYS REACHED"); // "REJECTED" will print AFTER this
})
.then(() => console.log("RESOLVED"))
.catch(() => console.log("REJECTED"));

Yes, the biggest difference is that reject is a callback function that gets carried out after the promise is rejected, whereas throw cannot be used asynchronously. If you chose to use reject, your code will continue to run normally in asynchronous fashion whereas throw will prioritize completing the resolver function (this function will run immediately).
An example I've seen that helped clarify the issue for me was that you could set a Timeout function with reject, for example:
new Promise((resolve, reject) => {
setTimeout(()=>{reject('err msg');console.log('finished')}, 1000);
return resolve('ret val')
})
.then((o) => console.log("RESOLVED", o))
.catch((o) => console.log("REJECTED", o));
The above could would not be possible to write with throw.
try{
new Promise((resolve, reject) => {
setTimeout(()=>{throw new Error('err msg')}, 1000);
return resolve('ret val')
})
.then((o) => console.log("RESOLVED", o))
.catch((o) => console.log("REJECTED", o));
}catch(o){
console.log("IGNORED", o)
}
In the OP's small example the difference in indistinguishable but when dealing with more complicated asynchronous concept the difference between the two can be drastic.

TLDR: A function is hard to use when it sometimes returns a promise and sometimes throws an exception. When writing an async function, prefer to signal failure by returning a rejected promise
Your particular example obfuscates some important distinctions between them:
Because you are error handling inside a promise chain, thrown exceptions get automatically converted to rejected promises. This may explain why they seem to be interchangeable - they are not.
Consider the situation below:
checkCredentials = () => {
let idToken = localStorage.getItem('some token');
if ( idToken ) {
return fetch(`https://someValidateEndpoint`, {
headers: {
Authorization: `Bearer ${idToken}`
}
})
} else {
throw new Error('No Token Found In Local Storage')
}
}
This would be an anti-pattern because you would then need to support both async and sync error cases. It might look something like:
try {
function onFulfilled() { ... do the rest of your logic }
function onRejected() { // handle async failure - like network timeout }
checkCredentials(x).then(onFulfilled, onRejected);
} catch (e) {
// Error('No Token Found In Local Storage')
// handle synchronous failure
}
Not good and here is exactly where Promise.reject ( available in the global scope ) comes to the rescue and effectively differentiates itself from throw. The refactor now becomes:
checkCredentials = () => {
let idToken = localStorage.getItem('some_token');
if (!idToken) {
return Promise.reject('No Token Found In Local Storage')
}
return fetch(`https://someValidateEndpoint`, {
headers: {
Authorization: `Bearer ${idToken}`
}
})
}
This now lets you use just one catch() for network failures and the synchronous error check for lack of tokens:
checkCredentials()
.catch((error) => if ( error == 'No Token' ) {
// do no token modal
} else if ( error === 400 ) {
// do not authorized modal. etc.
}

There's one difference — which shouldn't matter — that the other answers haven't touched on, so:
If the fulfillment handler passed to then throws, the promise returned by that call to then is rejected with what was thrown.
If it returns a rejected promise, the promise returned by the call to then is resolved to that promise (and will ultimately be rejected, since the promise it's resolved to is rejected), which may introduce one extra async "tick" (one more loop in the microtask queue, to put it in browser terms).
Any code that relies on that difference is fundamentally broken, though. :-) It shouldn't be that sensitive to the timing of the promise settlement.
Here's an example:
function usingThrow(val) {
return Promise.resolve(val)
.then(v => {
if (v !== 42) {
throw new Error(`${v} is not 42!`);
}
return v;
});
}
function usingReject(val) {
return Promise.resolve(val)
.then(v => {
if (v !== 42) {
return Promise.reject(new Error(`${v} is not 42!`));
}
return v;
});
}
// The rejection handler on this chain may be called **after** the
// rejection handler on the following chain
usingReject(1)
.then(v => console.log(v))
.catch(e => console.error("Error from usingReject:", e.message));
// The rejection handler on this chain may be called **before** the
// rejection handler on the preceding chain
usingThrow(2)
.then(v => console.log(v))
.catch(e => console.error("Error from usingThrow:", e.message));
If you run that, as of this writing you get:
Error from usingThrow: 2 is not 42!
Error from usingReject: 1 is not 42!
Note the order.
Compare that to the same chains but both using usingThrow:
function usingThrow(val) {
return Promise.resolve(val)
.then(v => {
if (v !== 42) {
throw new Error(`${v} is not 42!`);
}
return v;
});
}
usingThrow(1)
.then(v => console.log(v))
.catch(e => console.error("Error from usingThrow:", e.message));
usingThrow(2)
.then(v => console.log(v))
.catch(e => console.error("Error from usingThrow:", e.message));
which shows that the rejection handlers ran in the other order:
Error from usingThrow: 1 is not 42!
Error from usingThrow: 2 is not 42!
I said "may" above because there's been some work in other areas that removed this unnecessary extra tick in other similar situations if all of the promises involved are native promises (not just thenables). (Specifically: In an async function, return await x originally introduced an extra async tick vs. return x while being otherwise identical; ES2020 changed it so that if x is a native promise, the extra tick is removed where there is no other difference.)
Again, any code that's that sensitive to the timing of the settlement of a promise is already broken. So really it doesn't/shouldn't matter.
In practical terms, as other answers have mentioned:
As Kevin B pointed out, throw won't work if you're in a callback to some other function you've used within your fulfillment handler — this is the biggie
As lukyer pointed out, throw abruptly terminates the function, which can be useful (but you're using return in your example, which does the same thing)
As Vencator pointed out, you can't use throw in a conditional expression (? :), at least not for now
Other than that, it's mostly a matter of style/preference, so as with most of those, agree with your team what you'll do (or that you don't care either way), and be consistent.

An example to try out. Just change isVersionThrow to false to use reject instead of throw.
const isVersionThrow = true
class TestClass {
async testFunction () {
if (isVersionThrow) {
console.log('Throw version')
throw new Error('Fail!')
} else {
console.log('Reject version')
return new Promise((resolve, reject) => {
reject(new Error('Fail!'))
})
}
}
}
const test = async () => {
const test = new TestClass()
try {
var response = await test.testFunction()
return response
} catch (error) {
console.log('ERROR RETURNED')
throw error
}
}
test()
.then(result => {
console.log('result: ' + result)
})
.catch(error => {
console.log('error: ' + error)
})

The difference is ternary operator
You can use
return condition ? someData : Promise.reject(new Error('not OK'))
You can't use
return condition ? someData : throw new Error('not OK')