I am struggling to fix what I think is a very simple jQuery promise problem. In effect, I need to bypass a 'then' in certain circumstances within a promise chain when a previous promise discovers it won't fire asynchronously.
I have a chunk of code (the actual, runtime code is too long to repost here coherently - apologies/mea culpa in advance) for which the "then" portion may run code that will fire asynchronously, or will complete immediately and return - no async execution.
***This has been resolved. In digging through the code, I happened to discover a spot where a promise was not being properly resolved in the no-files condition. This has fixed the problem. Thanks for the keen eyes to #Bergi that indicated something was really awry.
The overarching structure of the code is in this fashion:
****Edited to correct improper call to DisableProgress**
function writeData(){
EnableProgress('Starting upload')
.then(writeRecord)
.then(uploadFiles)
.then(DisableProgress);
}
function uploadFiles(){
var deferred = $.Deferred();
if (fileUploadList.length==0){
deferred.resolve();
} else {
//perform async upload
}
return deferred.promise();
}
The problem is this: when uploadFiles detects no files to upload (a perfectly legitimate situation), no async process occurs; the promise is resolved before it is returned, and the 'then' portion that calls 'DisableProgress' never fires. (There are actually other calls in this chain that fired but are eliminated here for clarity's sake)
I thought about putting in a brief setTimeout within the uploadFiles routine to "force" the async execution in the event there were no files to upload, but that seemed an absurd hack to me. Surely there's an obvious/better way I'm simply overlooking. I've searched Google about issues revolving around resolving a promise prior to its return, but I've not seen (or recognized?) anything on point. I think I'm at a point where I can't see the trees for the forest - I'm making it too complicated. What is my error?
Related
I'm given a promise and am returning it at the end of my function for other code to do more actions after me. A kind of middleware, if you will.
return apiCallPromise;
Lets say for some reason I wish to stop those actions from happening and "swallow" a particular promise. For example by doing the following:
if (promiseShouldBeStopped) return new Promise((resolve) => {})
return apiCallPromise;
This correctly returns a promise, however it never resolves. And so the 'promise chain' has been stopped dead in its tracks.
This seems like an anti-pattern to me as it the promise is left pending forever. Are there any bad effects of this that I should be aware of, or is this a fine technique for "filtering" promises?
Attempt to answer my own question
Does this cause memory issues?
Future code doesn't know that this promise has been filtered out, and so fails to do any "catch" or "finally" cleanup code.
In my case this is not relevant, and I want this code to take care of those errors so that the future code doesn't have to deal with them. (this means I can guarantee that onFulfilled gives the api response, and onRejected only gives an error that is actually unexpected)
I am confused with certain parts regarding promises, I have read multiple articles and I have seen multiple videos and I want to ask a few things:
from what I understand currently, when a promise is created is starts running.
Do I need to await on the promise if the value that returned from the promise is not used in the code ?
a scenario for that is: let's say I am processing a task in my system and I want to log the information to mongodb, when I invoke the insert function I get a promise back. the execution beings but I don't care about the result of it
if I am not awaiting and there is an error I wouldn't be able to handle it.
A followup question to the question above:
from what I read whenever I await it actually blocks the execution of the async function, but if it blocks the execution of the function how doesn't it block the rest of the eventloop ?
Basic concepts
The whole point of the event loop is to have many microtasks that do not affect each other (hence by default there is no effect).
To chain microtasks first; there were callbacks, then Promises (then/catch) then the async/await API. The last two can be considered just syntactic sugar atop the callback concept. There are no 'functionalities' added but rather a different syntax to achieve the same stuff in simpler and more elegant ways (Pyhilosophicaly).
The event loop executes all the queued microtasks at each loop and repeats. Unless you have blocking code (and await is not to be considered blocking) your event loop never stalls and hence other tasks are not affected.
You are trying to understand await from the perspective of real blocking code as intended in other languages.
IMHO you first need to deeply understand how callbacks work, then study Promises (as a tool to make callbacks less messy) and then async/await (as a syntax to make Promises pretties). But keep in mind, the underlying system is the same: functions that call functions that get handled functions to be eventually called in future).
Specific questions
When a promise is created is starts running
Yes, but no. A promise does not run, a promise is only a contract you receive by a part of code that will be used to notify you of the outcome. So the promise does not run, is the mean that has been created for you after you requested a task to be executed.
So typically if a promise has been handled to you there is something 'running'. But Promise may be used differently and there may be something 'waiting'.
A promise is not linked to the execution of the task hence it can not start nor stop it.
Do I need to await on the promise if I'm not interested in the outcome
No, you are not required to. But keep in mind that not handling promise exceptions is being deprecated and may result in system failure. You should always handle (or let bubble) exceptions.
There would be a failure if there is an unhandled promise rejection. In synchronous code this is equivalent to an uncaught thrown error. Until now(-ish) uncaught promise rejections were tolerated but there isn't a really good reason for that. Node is moving to treat them the same as any other error that bubbles to the top.
VLAZ
You are considering promises only with async/await but the underlying Promise api is .then() and .catch(). Using this API you can use promises in a 'fire-and-forget' fashion:
async function Do() {
await before();
asyncDbCall().catch(err => console.error(err))
await after();
}
In this example you are not waiting for asyncDbCall() but still .catch(err => console.error(err)) will result in the error being logged (some time in the future, probably even after Do() has completed).
Or you can branch off the execution to other async executions, take this complex example:
async function Do() {
await before();
// This will execute AFTER before() & only if before() succeeded
asyncDbCall()
.then(async value => {
// This will execute after `asyncDbCall()` and only if it succeeded
await something(value);
// We reach here after `something()` and only if succeeded
})
.catch(err => {
// This will execute if `asyncDbCall()` fails of IF ANYTHING
// within `async value => {}` fails
console.error(err);
})
// This will execute AFTER before() and only if before() succeeded and
// asyncDbCall() call (but not Promise) succeeded
await after();
}
Await it actually blocks the execution of the async function
Await stops the async function (hence also anything that is awaiting for the function) but does not affect anyway the event loop.
from what I understand currently, when a promise is created is starts running.
It is not. It has its internal state set to pending. Promise's constructor takes a callback as an argument, and in turn provides it with resolve and reject callbacks.
What it also does is that it allows to provide a number of actions that
happen when it's state changes to resolved or rejected. Outside of async/await, you might know them as .then and .catch instance methods of Promise class. Once the state is changed, they will be executed.
Do I need to await on the promise if the value that returned from the promise is not used in the code?
No, that is entirely up to you.
a scenario for that is: let's say I am processing a task in my system and I want to log the information to mongodb, when I invoke the insert function I get a promise back. the execution beings but I don't care about the result of it
if I am not awaiting and there is an error I wouldn't be able to handle it.
You can still use .catch to handle the error without awaiting for the Promise to finish
A followup question to the question above:
from what I read whenever I await it actually blocks the execution of the async function, but if it blocks the execution of the function how doesn't it block the rest of the eventloop?
Promises have nothing to do with the event loop.
You can read more about the EventLoop here.
I want to be able to reject and stop the rest of the promise from running if an external event occurs. This is the example usecase:
new Promise((resolve,reject)=>{
websocket.onerror=reject;
//do processing here
websocket.onerror=undefined;
resolve(...);
});
This has the intended effect of rejecting the promise if an error occurs while the promise is running. However, the problem is that the rest of the promise will continue running even if a rejection occurs.
Using async/await the above is not possible at all.
I think one way would be to have every single line of code in a separate promise, and cancelling the chain on an event, but that would be a pain.
However, the problem is that the rest of the promise will continue running even if a rejection occurs.
This is a basic misunderstanding (and you're not alone in it!). There is no "rest of the promise." A promise is just a means of observing the completion of an asynchronous process. It's the process, not the promise, that's continuing.
How you cancel that process (and whether you can) depends entirely on what that process is and whether it provides a means of cancelling it, which is unrelated to promises themselves. You seem to be using a web socket, so you'd need to use the web socket API to cancel whatever you have it doing (or to send a message to the other end to tell it to stop what it's doing).
For instance: Suppose the promise were waiting for a timer (setTimeout) to fire, and would resolve when it did. Then some other thing happens and you no longer want that. Rejecting the promise just rejects the promise, it doesn't have any effect on the timer. You'd need to cancel the timer, too (clearTimeout).
Re your edit:
I think one way would be to have every single line of code in a separate promise, and cancelling the chain on an event, but that would be a pain.
No, the code in the promise executor (the function you pass new Promise) is run synchronously. Again, there's no "rest of the code" to cancel. There may well be asynchronous callbacks in there, or a process the code starts you need to send a cancellation too, but that's not going to require putting every line in its own promise, not at all.
Understanding the answer to this question may help you:
When you create the promise, it is executed synchronously. So this line is executed:
websocket.onerror=reject;
At this point, your code begins to process. If there is an event of websocket error, it is put in the js event loop, but your promise is still executing synchronously. Then you unhook the websocket's error handler, and resolve. The promise will resolve even if there was an error. At this point the call for resolve is also added to js' event loop. Then the next js tick executes, and maybe reject is called, maybe not, but it doesn't matter at this point because you resolved synchronously, and promises never change their minds once they've resolved or rejected.
So the key takeaway here is that creating a promise executes it synchronously. It's not a thread that you can cancel while waiting for other events. The promise' resolution or rejection is executed asynchronously, but that doesn't help you at all.
To cancel a promise, you'll need some pretty basic stuff such as
if(someOtherVariableThatDeterminesCancelation){
reject()
}
I'm having trouble controlling execution flow. This is a follow-on to node.js, bluebird, poor control of execution path and node.js table search fails with promises in use. Judging by console.log print-outs, my recursive routine works great, except that the first call to resolve() (a signal to the nth recursive call) gives the green light to follow-on code that shouldn't get that green light until the first call to the recursive routine calls resolve(). It turns out the first call to the recursive routine delivers the answer I want reported, but by the time it reports it, the follow-on code is no longer listening for it and is running blissfully along with an "undefined" answer. Bad.
My code is much to long to share here. I tried to write a small model of the problem, but haven't found the combination of factors to replicate the behavior.
Sound familiar? How do you keep proper control over Promises releasing follow-on code on time?
I thought maybe the first call to the routine could start an array passed into a Promise.all and later calls would add another entry to that array. I haven't tried it. Crazy?
Without seeing your actual code, we can't answer specifically.
Sound familiar? How do you keep proper control over Promises releasing
follow-on code on time?
The answer is always to not resolve the first promise in the chain until you're ready for things to execute and to structure your promise chain so that dependent things don't get executed until the things they are waiting on have been properly resolved. If something is executing too soon, then you're either calling something too soon or your promise structure is not correct. Without seeing your actual code, we cannot know for sure.
A common mistake is this:
someAsyncOperation().then(someOtherAync()).then(...)
which should be:
someAsyncOperation().then(someOtherAync).then(...)
where you should pass a reference to the next async function rather than calling it immediately and passing its return value.
I thought maybe the first call to the routine could start an array
passed into a Promise.all and later calls would add another entry to
that array. I haven't tried it. Crazy?
You cannot pass an array to Promise.all() and then add things to the array later - that is not a supported capability of Promise.all(). You can chain subsequent things onto the results of Promise.all() or do another Promise.all() that includes the promise from the previous Promise.all() and some more promises.
var someArrayOfPromises = [...];
var pAll = Promise.all(someArrayOfPromises);
var someMorePromises = [...]
someMorePromises.push(pAll);
Promise.all(someMorePromoises).then(...)
We have a class as part of a large framework, and we use Q as our promise library.
We have a function similar to what you see below.
this._getData() returns a promise, and all of the other functions in the 'thens' receive and return a value properly.
Everything works most of the time.
Sometimes in Safari 6.1 + and 7+ (mainly when the data is not yet in the cache, i.e. the first time anyone ever requests it), the first promise (this._getData()) is created and fulfilled (I created a local log to ensure this), but none of the 'thens' are called. Q does not pass the fulfillment value to the next task.
There was a bug that looked similar to it in Q v0.8 which was fixed in v0.9, but we upgraded to v0.9 and it has not resolved the issue.
Here's the code:
someAsyncFunc: function(){
var promise = this._getData()
.then(this._doSomething_1)
.then(this._doSomething_2)
.then(...)
.then(...)
.then(this._doSomething_n)
.fail(this._onFail);
return promise;
}
It's interesting to note that once the bug happens, Q is completely broken. It may have been broken before this request due to something else that failed which uses Q, but we don't see any exceptions.
By completely broken, I mean that normally on any page which has Q (and when the bug does not occur), if you type this into the console:
Q(1).then(function(){console.log('then has occurred');});
then you will see the console log.
However, when the bug occurs, this does not happen.
Even trying:
var promise = Q(1);
promise.then(function(){console.log('then has occurred');});
//nothing happens
promise.isFulfilled() //true
Which seems to show that Q is just stuck in some weird state where it believes it is flushing when it's really not, so it never goes to the next task.
This is actually exactly what we're experiencing, as the this._getData() creates a new promise, performs an ajax call to the server and passes the deferred object to resolve into the callbacks, and successfully resolves the promise - but none of the next functions are called.
Would love to know if anyone encountered this and knows how to fix it, or has an idea how to continue debugging this.
Thanks
Update: After further debugging, it definitely looks like Q gets stuck flushing on a task, and is therefore never able to move to the next task.