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May this long polling code cause stackoverflow? [duplicate]

For example I found some api library that is based on promises, and I need to issue api requests using this library in some interval, infinite times (like usual back-end loop). This api requests - actually chain of promises.
So, if I write function like:
function r(){
return api
.call(api.anotherCall)
.then(api.anotherCall)
.then(api.anotherCall)
...
.then(r)
}
Will it cause stack overflow?
Solutions that I come up with is to use setTimeout for a call of r recursively.
function r(){
return api
.call(api.anotherCall)
.then(api.anotherCall)
.then(api.anotherCall)
.then(()=>{setTimeout(r, 0)})
}
So setTimeout will call r actually only when call stack is empty.
Is it good solution, or there is some standard way of calling promises recursively?

Will this cause stackoverflow?
No, it will not. Per the promise specification, .then() waits for the stack to completely unwind and is then called after the stack is clear (essentially on the next tick of the event loop). So, .then() is already called asynchronously after the current event is done processing and the stack is unwound. You do not have to use setTimeout() to avoid stack build-up.
Your first code example will not have any stack build-up or stack overflow, no matter how many times you repeat it.
In the Promises/A+ specification, section 2.2.4 says this:
2.2.4 onFulfilled or onRejected must not be called until the execution context stack contains only platform code. [3.1].
And, "platform code" is defined here in 3.1:
“platform code” means engine, environment, and promise implementation code. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop turn in which then is called, and with a fresh stack. This can be implemented with either a “macro-task” mechanism such as setTimeout or setImmediate, or with a “micro-task” mechanism such as MutationObserver or process.nextTick. Since the promise implementation is considered platform code, it may itself contain a task-scheduling queue or “trampoline” in which the handlers are called.
The ES6 promise specification uses different words, but generates the same effect. In ES6, promise .then() is performed by enqueing a job and then letting that job get processed and the job only gets processed when no other code is running and the stack is empty.
This is how running such as job is described in the ES6 spec:
A Job is an abstract operation that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.
Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 25. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.

Recursive promises can cause stack overflow?

For example I found some api library that is based on promises, and I need to issue api requests using this library in some interval, infinite times (like usual back-end loop). This api requests - actually chain of promises.
So, if I write function like:
function r(){
return api
.call(api.anotherCall)
.then(api.anotherCall)
.then(api.anotherCall)
...
.then(r)
}
Will it cause stack overflow?
Solutions that I come up with is to use setTimeout for a call of r recursively.
function r(){
return api
.call(api.anotherCall)
.then(api.anotherCall)
.then(api.anotherCall)
.then(()=>{setTimeout(r, 0)})
}
So setTimeout will call r actually only when call stack is empty.
Is it good solution, or there is some standard way of calling promises recursively?

Will this cause stackoverflow?
No, it will not. Per the promise specification, .then() waits for the stack to completely unwind and is then called after the stack is clear (essentially on the next tick of the event loop). So, .then() is already called asynchronously after the current event is done processing and the stack is unwound. You do not have to use setTimeout() to avoid stack build-up.
Your first code example will not have any stack build-up or stack overflow, no matter how many times you repeat it.
In the Promises/A+ specification, section 2.2.4 says this:
2.2.4 onFulfilled or onRejected must not be called until the execution context stack contains only platform code. [3.1].
And, "platform code" is defined here in 3.1:
“platform code” means engine, environment, and promise implementation code. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop turn in which then is called, and with a fresh stack. This can be implemented with either a “macro-task” mechanism such as setTimeout or setImmediate, or with a “micro-task” mechanism such as MutationObserver or process.nextTick. Since the promise implementation is considered platform code, it may itself contain a task-scheduling queue or “trampoline” in which the handlers are called.
The ES6 promise specification uses different words, but generates the same effect. In ES6, promise .then() is performed by enqueing a job and then letting that job get processed and the job only gets processed when no other code is running and the stack is empty.
This is how running such as job is described in the ES6 spec:
A Job is an abstract operation that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.
Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 25. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.

Inconsistent interplay between IndexedDB transactions and Promises

I saw sync-promise posted on Reddit and got into a discussion with the author. We noticed some weird inconsistencies in the relationship between IndexedDB transactions and promises.
IndexedDB transactions auto-commit when all the onsuccess events finish. One complication is that you can't do anything asynchronous inside an onsuccess callback except do another operation on the same transaction. For example, you can't start an AJAX request in an onsuccess and then reuse the same transaction after the AJAX request returns some data.
What do promises have to do with it? As I understand it, promise resolution is supposed to always be asynchronous. This would imply that you can't use promises without auto-committing an IndexedDB transaction.
Here is an example of what I'm talking about:
var openRequest = indexedDB.open("library");
openRequest.onupgradeneeded = function() {
// The database did not previously exist, so create object stores and indexes.
var db = openRequest.result;
var store = db.createObjectStore("books", {keyPath: "isbn"});
var titleIndex = store.createIndex("by_title", "title", {unique: true});
var authorIndex = store.createIndex("by_author", "author");
// Populate with initial data.
store.put({title: "Quarry Memories", author: "Fred", isbn: 123456});
store.put({title: "Water Buffaloes", author: "Fred", isbn: 234567});
store.put({title: "Bedrock Nights", author: "Barney", isbn: 345678});
};
function getByTitle(tx, title) {
return new Promise(function(resolve, reject) {
var store = tx.objectStore("books");
var index = store.index("by_title");
var request = index.get("Bedrock Nights");
request.onsuccess = function() {
var matching = request.result;
if (matching !== undefined) {
// A match was found.
resolve(matching);
} else {
// No match was found.
console.log('no match found');
}
};
});
}
openRequest.onsuccess = function() {
var db = openRequest.result;
var tx = db.transaction("books", "readonly");
getByTitle(tx, "Bedrock Nights").then(function(book) {
console.log('First book', book.isbn, book.title, book.author);
return getByTitle(tx, "Quarry Memories");
}).then(function(book) {
console.log('Second book', book.isbn, book.title, book.author);
// With native promises this gives the error:
// InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable
// With bluebird everything is fine
});
};
(Full disclosure: demo was created by paldepind, not me!)
I've tried it in Chrome and Firefox. It fails in Firefox due to the transaction auto-committing, but it actually works in Chrome! Which behavior is correct? And if Firefox's behavior is correct, is it literally impossible to use "correct" promise implementations with IndexedDB transactions?
Another complication: If I load bluebird before running the above demo, it works in both Chrome and Firefox. Does this imply that bluebird is resolving promises synchronously? I thought it wasn't supposed to do that!
JSFiddle

This is probably due to the difference between microtasks and tasks ("macrotasks"). Firefox has never had a standards-complaint promise implementation that uses microtasks, whereas Chrome, Bluebird, and others correctly use microtasks. You can see this in how a microtask (which executes "sooner" than a macrotask, but still async) falls inside the transaction boundary, whereas a macrotask (e.g. from Firefox's promises) does not.
So, this is a Firefox bug.

Ok, so I've once again taken a deep dive into the IndexedDB, the DOM and the HTML specification. I really need to get this right for SyncedDB since it relies heavily on promises inside transactions.
The crux of the problem is whether or not the delayed execution of the onFulfilled and the onRejected callbacks to then that Promises/A+ compliant must exhibit will trigger an IndexedDB transaction commit.
The IndexedDB rules for a transactions lifetime are actually pretty straight forward when you extract them from the specification and line them up:
Request can only made against a transaction when its active flag is set to true (as specified here).
When a transaction is created it is initially active until control is returned to the the browsers event loop (this is specified in the transaction creation steps).
Every time a success or error event is fired, the transactions active flag is set to true before as the last step before the event is dispatched. After the event dispatch the transaction is flagged as inactive again (this is specified in the steps for firing a success/error event.
When a transaction can no longer become active it will automatically be committed (as specified here).
This roughly translates to:
When you create a transaction you can place as many request against it as you wish.
From then on new request can only be made inside event handlers for another requests success or error event listener.
When all requests has been executed and no new requests placed the transaction will commit.
The question then becomes: If a promise is fulfilled inside a request's success or error event listener will its onFulfilled callbacks be invoked before the IndexedDB sets the transaction as inactive again? I.e. will onFullfilled callbacks be called as part of step 3 in firing a success event?
The step dispatches an event and IndexedDB uses DOM events so the actual operation performed is beyond the IndexedDB specification. The steps for dispatching an event is, instead, specified here in the DOM specification. Going over the steps it becomes clear that at no point is a microtask (which would call the promise callbacks) checkpoint performed. So the initial conclusion is that the transaction will be closed before any onFulfilled callbacks will be invoked.
However, if we attach the event listeners by specifying an onsuccess attribute on the request object things gets more hairy. In that case we are not simply adding an event listener as per the DOM specification. We are instead setting an event handler IDL attribute as defined in the HTML specification.
When we do that the callback is not added directly to the list of event listeners. It is instead "wrapped" inside the the event handlers processing algorithm. This algorithm performs the following important operations:
In step 3 it runs the jump to code entry-point algorithm.
This then performs the steps to clean up after running a callback which
Finally, this performs a microtask checkpoint. Which means that your promise callbacks will be invoked before the transaction is marked as inactive! Hurrah!
This is good news! But it is weird how the answer depends on whether or not you listen for the success event by using addEventListener or set a onsuccess event handler. If you do the former the transaction should be inactive when your promise's onFulfilled callbacks is invoked and if you do the later it should still be active.
I was, however not able to reproduce the difference in existing browsers. With native promises Firefox fails at the example code no matter what and Chrome succeeds even when using addEventListener. It is possible that I've overlooked or misunderstood something in the specifications.
As a final note Bluebird promises will close transactions in Internet Explorer 11. This is due to the scheduling that Bluebird uses in IE. My synchronized promise implementation works inside transactions in IE.

You are correct: Promises are resolved asynchronously, and IndexedDB has some synchronous requirements. While other answers point out that native promises may work correctly with IndexedDB in certain versions of certain browsers, it is likely as a practical matter that you will have to deal with the issue of it not working in some of the browsers you're targeting.
Using a synchronous promise implementation instead, however, is a horrible idea. Promises are asynchronous for very good reasons, and you are introducing needless chaos and potential for bugs if you make them synchronous instead.
There is a fairly straightforward workaround, however: use a Promise library that provides a way to explicitly flush its callback queue, and an IndexedDB wrapper that flushes the promise callback queue after invoking event callbacks.
From the Promises/A+ point of view, there isn't any difference between the handlers being called at the end of the event, or at the beginning of the next tick cycle -- they are still being called after all the code that set up the callbacks has finished, which is the important part of Promise asynchrony.
This allows you to use promises that are asynchronous, in the sense of meeting all the Promises/A+ guarantees, but which still ensure that the IndexedDB transaction isn't closed. So you still get all the benefits of callbacks not happening "all at once".
The catch of course is that you need libraries that support this, and not every Promise implementation exposes a way to specify a scheduler or to flush its callback queue. Likewise, I'm not aware of any open source IndexedDB wrappers that have support for this.
If you are writing your own IndexedDB wrapper with Promsies, though, it would be good for it to use an appropriate Promise implementation, and flush its callback queue accordingly. One easy option would be to embed one of the many "micropromise" implementations that are only 100 lines or so of Javascript, and modify it as needed. Alternately, using one of the larger mainstream Promise libraries with custom scheduling support would be doable.
Do not use a synchronous promise library, the synchronous Bluebird build, or a synchronous scheduler. If you do that, you might as well abandon promises altogether and use straight callbacks.
Follow-up note: one commenter suggests that a synchronous promise is as safe as flushing a callback queue. But they are wrong. Horribly, horribly wrong. You can reason about a single event handler well enough to say "there isn't any other code running here; it's okay to invoke the callbacks now". To make a similar analysis with synchronous promises requires a complete understanding of how everything calls everything else... which is precisely opposite from the reason you want promises in the first place.
In the specific sync-promise implementation, the sync-promise author claims that their promise library is now "safe", and doesn't "release Zalgo". They are once again wrong: it isn't safe, and does release Zalgo. The author apparently did not actually understand the article about "releasing Zalgo", and has successfully reimplemented jQuery promises, which are widely considered horribly broken for a number of reasons, including their Zalgo-ness.
Synchronous promises are simply not safe, no matter your implementation.

Is using timers in deferred/promises implementation an evil?

Many of my friends, who are using deeply some deferred/promises objects in their libraries, are often telling me, that to use timers in own implementation of it is an evil.
That it doesn't correspond to A+: https://github.com/promises-aplus/promises-spec
And that many libraries as jQuery and others don't use timers. So I've tried to find any timers in jQuery sources, which may relate to promises implementation, but no success:
https://github.com/jquery/jquery/blob/master/src/deferred.js
All, right, but I've found some notes in A+ description, which have confused me about using timers in it:
At Notes article:
Here "platform code" means engine, environment, and promise
implementation code. In practice, this requirement ensures that
onFulfilled and onRejected execute asynchronously, after the event
loop turn in which then is called, and with a fresh stack. This can be
implemented with either a "macro-task" mechanism such as setTimeout or
setImmediate, or with a "micro-task" mechanism such as
MutationObserver or process.nextTick. Since the promise implementation
is considered platform code, it may itself contain a task-scheduling
queue or "trampoline" in which the handlers are called.
So, I understood A+ didn't have strict rules about timer using or did it?
Help me, I'm rather confused.

You are confusing the use of setTimeout with "setting a timer" - Promises/A+ implementations typically use setTimeout to guarantee asynchronous execution of handler functions, not to delay execution by some time period.
Promises/A+ guarantees that the fulfilled and rejected methods are called asynchronously, regardless of when the promise is fulfilled. One way to guarantee async execution in a browser JS environment is to wrap a function call in setTimeout with a timeout of zero (the default).
jQuery does not guarantee asyc execution of fulfilled/rejected callbacks (which is a major design flaw), so an async wrapper call is not required.

Using timers in general in your applications is a bad practice (other than scheduling tasks, that is).
You can never be sure how long an action will take. So you end up doing one of three things:
You either allocate not enough time, in which case, stuff breaks because some things you expected to happen hadn't happened yet.
Or you allocate too much time, in which case you application is slow for no good reason
Or, worst case, you allocate just enough time, which causes your application to sometimes break, and sometimes work as expected.
I'm not sure about specs and stuff. But using timers and delays in your application in general is a bad idea.

Javascript Promises library to make "long-running-code-non-blocking-UI" in browser?

Update
this an update to the question below and should help finding an answer
Taking up the answer from torazaburo who also quoted part of the prominent Javascript Promise/A+ definition I want to update the question here.
The Promise/A+ specification suggest in point 2.2.4 this:
onFulfilled or onRejected must not be called until the execution
context stack contains only platform code. 3.1.
and further explains
Here “platform code” means engine, environment, and promise
implementation code. In practice, this requirement ensures that
onFulfilled and onRejected execute asynchronously, after the event
loop turn in which then is called, and with a fresh stack. This can be
implemented with either a “macro-task” mechanism such as setTimeout or
setImmediate, or with a “micro-task” mechanism such as
MutationObserver or process.nextTick. Since the promise implementation
is considered platform code, it may itself contain a task-scheduling
queue or “trampoline” in which the handlers are called.
The very issue I look forward to find with this question is having as the crucial point that Promise implementation Javascript code is itself considered platform code and allows to not yield to the eventloop inbetween resolving subsequent promise via calling the onFulfilled onRejected functions associated. This is good in Node.js(server) as it avoids unnessary relinguishing back to the event-loop (leaving the execution stack), but also causes the challange in a Browser that since the execution stack is not exited in between resolving a potentially large number of Promises (which themselves can generate new Promises). Not leaving the execution stack and yielding to the event loop is causing in a Browser the undesired (blocking script warning/problem).
The "trampoline" task-scheduling of the Promise implementation which causes this needs however not necessary refrain from handing back the execution to the Javascript event loop from time to time. Such a feature would allow for using Promises for heavier tasks. Such an implementation for Promises for "long-running-code" is searched/asked for in this question.
Clarification: The "excessive lenght" is not the individual length of the onFulfilled function, but the joining together several those functions/callbacks as result of the Promise resolving process (when done in such a "trampoline" way). I am already aware that if one individual onFulfilled funciton is too long, this cannot be helped in any way by using any sort of Promise implementation.
The deal here is that the subsequent resolvement of x promises (within one excecution stack and hence without handing back to the Javascript event loop) can provoke an excessive length duration of Javascript code execution. This, when in a Browser is bad (because of blocking).
The question
In Javascript, Promises allow to deal with asynchronous programming tasks. Great!
There are already some implementations and libraries arround Q, WinJS or when.js to name just a few.
Having looked at then I see that they tackle some of the "special things" in Javascript asynchronous programming challanges.
Normally I perceive them to do this for promise resolution
Go to the internal list of promises
Check if the promise is fullfilled + run all the associated (via then(onFullfilled,onReject)) functions.
(in some cases we are done here)
(in other cases there will be still "pending" promises)
This case (4) is because to have them (the remaining promises) fullfilled would need the current Javascript Code (which is this very code for promise resolution) to stop running and allow JS event loop to happend (i.e asynchronous things like XHR-requests, or User-UI-interaction). To make this (4) work, the promise resolution normaly schedules a recall (i.e. via setTimeout/setImmediate) and continues after the event loop ran and hence maybe some of the "pending" promises have been settled (=rejected/fullfilled).
My worry is that the step 1 and 2 could be runnning for quite a some time, only releasing execution to the event loop in case it seems indicated to settle some of the "pending" promises. While "okay" in some cases (i.e. on the server/Node.js) it is quite problematic in a browers, because even though it was no problem to release execution to the event loop and have the UI not-blocking, this is not done in the implementations of promises I have seen.
My question therefore is:
Do you know a promise implementation (Javascript Promises library) that cares for the aspect:
to make "long-running-code-non-blocking-UI" in browser?
which would mean that the promise resolution would voluntarily release execution back to the event loop so that CSS animations, user input, mouse interaction, does get enough attention and that there will be no "Warning: Unresponsive script" message.

Any compliant promises implementation will not run the then functions synchronously, but rather only at the next tick. Therefore, your worry that "step 1 and 2 could be runnning for quite a some time" is unfounded.
From the Promises/A+ spec:
onFulfilled or onRejected must not be called until the execution context stack contains only platform code.
Here "platform code" means engine, environment, and promise implementation code. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop turn in which then is called, and with a fresh stack.
In other words, your formulation under 2) is incorrect. The promises implementation does not "run the associated functions", it schedules them.
This cannot help you--indeed there is no way to help you--if a handler itself is "long-running" code.

I think the solution could be to parse that long-running JavaScript code for example with https://github.com/NeilFraser/JS-Interpreter.
It will make the code be even slower, but you could specify the priority:
const myInterpreter = new Interpreter(myCode);
function nextStep() {
if (myInterpreter.step()) {
window.setTimeout(nextStep, 100/speed);
}
}
nextStep();