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Convert synchronous Ajax calls to asynchronous

I have hundreds of master JS scripts which look like this and which I may not modify:
> some code filling parameter1
var response1=callAjax(parameter1);
> some code using response1 and filling parameter2
var response2=callAjax(parameter2);
> etc ..
The callAjax function can be changed or re-written. It currently calls Ajax in synchronous mode - which is deprecated - so it needs to be overhauled. Blocking the UI during the execution of the master file is a requirement (these are banking and similar applications so the end user is not allowed to edit any input fields or click any button during the process - a screen veil can be shown).
How can I modify the callAjax function to use async mode and leave the master scripts (hundreds or thousands of them) un-changed ?
I looked at using Promises or async/await but cannot figure how to use these in order to return the ajax response to the master scripts. Same with adding some global variables used as semaphores. Again, breaking the master scripts into several functions for each call is not allowed. These are plain Javascript, no jQuery.

How can I modify the callAjax function to use async mode and leave the master scripts (hundreds or thousands of them) un-changed ?
You can't. If you use asynchronous ajax, then you can't return the value from the function because the function will return BEFORE the value is available. You simply can't make an async operation into a synchronous function. For a lot more detail on returning asynchronous operations from a function, see How do I return the response from an asynchronous call?. You will see that all the available options involve calling a callback when the async value is done or returning a promise and using a callback with .then() on the promise to know when the value is done and what it is.
If your requirement is that you block the UI and that you can't change the calling code, then you have to stick with synchronous Javascript. No asynchronous operation will do either.
Otherwise, get rid of those requirements and write proper asynchronous calling code and don't block the UI in this way (you can protect the contents of the screen from editing or button presses during the ajax operation with different techniques).
So, my conclusion from the problem you've described is that you need to lift some of your current requirements because there is no solution that:
Blocks the UI
Uses only asynchronous Ajax
Returns the ajax obtained value directly from callAjax()
Does not change the calling code that calls callAjax()
You can't do all of those at once.
The forward moving design choice would be to return a promise from callAjax() and fix all the calling code that uses it to use that promise. For blocking the UI, you'd have to use some sort of UI protection (screen veil, you called it, or something like that).
My advice if you have a large body of code using this old synchronous ajax function is to create a new ajax function with a different name that returns a promise and deprecate use of the old one for any new code (make all new code use the new one). Then, as needed and guided by business goals, rewrite uses of the old function to use the new one one at a time. This will at least stop more code being written with the old, synchronous ajax code. And, it will give you an opportunity over time to convert code to the new interface as needed.
Note that the closest thing you could get to calling code that looks close to what you have (but still requires changes) would be using async/await (available in the ES7 specification or in transpilers). You'd return a promise from callAjax() and then, the caller can await the result:
// async function returns a promise
async function someFunction() {
try {
// some code filling parameter1
var response1 = await callAjax2(parameter1);
// some code using response1 and filling parameter2
var response2 = await callAjax2(parameter2);
// etc...
} catch(e) {
// handle error here
}
}
P.S. It looks like your current code doesn't have a good way to communicate back ajax errors.

JavaScript native Promise execute callback on both results

Is there any way to execute callback on both results of Promise object?
For example I want to make some cleanup logic after execution of xhr request. So I need to do something like this:
var cleanUp = function() { something.here(); }
myLib.makeXhr().then(cleanUp,cleanUp);
In jquery Defered for example i can use method always():
myLib.makeXhr().always(function() { something.here(); });
Does Promise support something like this?

No, there is none. It was discussed but the spec is minimal. It doesn't include a bunch of other functionality. It's designed to interoperate well with library promises and to provide simple functionality.
Here is a correct polyfill of that proposal originally made by StefPanner.
Moreover, I disagree with the current now deleted answers adding it themselves because they're all doing it wrong (as an enumerable property - no fun). Even if we ignore what it does to the return values and error state of the returned promise. The intended way to extend native promises is by subclassing them, sadly, no browsers support this yet so we'll have to wait.
Instead of messing with native prototypes, we should use a different pattern:
openDb().then(foo).then(bar).finally(close).then(more);
Is susceptible to us forgetting to call close, even if we open it 100 times in our app, forgetting to close it even once can still be devastating. On the other hand - we can use the disposer pattern which some promise libraries provide built in ourselves:
openDb(function(db){
return foo(db).then(bar);// chain here
}).then(more);
Basically - this pattern means instead of having openDB return a promise - we have it take a function and return a promise, when the function is run, if it returns a promise we wait for that promise to resolve. It looks something like:
function openDb(withDb){
return justOpenWithoutCleanUp().
then(withDb).
then(clean, function(e){ clean(); throw e; }); // note the rethrow
}

Promise object supports 'always'.
For eg:
var oPromise= jQuery.ajax({
url:YOUR_URL
}).always(function(){
YOUR_CLEAN_UP_METHOD();
})

Asserting values in node.js

I have a function,
Edit1 - Updated function with real one because the previous one was simplified synchronous function and the code would have worked as correctly pointed by #AlexMA in the comments
'returnSuccessOrFailure': function () {
return driver.findElement(wd.By.css('div#button')).then(function (button) {
return button.getAttribute('class').then(function (status) {
return status;
});
});
}
In my node.js test, my assertion is failing because the assert is called before returnSuccessOrFailure finishes execution.
var value = returnSuccessOrFailure();
assert.equal(value,'success', 'Looks like something failed');
If I implement a promise in returnSuccessOrFailure and chain my assert then that works. My question is do I have to implement promises all the time for such situations to block the execution? I am new to Javascript and the async nature of it and any insight when to use promises and when not to would be useful.

you don't have to "implement a promise" in, just return the one you already have:
returnSuccessOrFailure': function () {
return driver.findElement(wd.By.css('div#button')).then(function (button) {
...
but then, yes, you do still need to put your assert in a done handler
returnSuccessOrFailure().done(function(value) {
assert.equal(value,'success', 'Looks like something failed');
}

Chaining you asserts will not only make it work but will also make for more readable code. Knowing what happens in what order can be useful when going back to refactor. Not only that but the structure of callbacks/promises also allow for easily written timer tests.
Also, since your test needs to have the current state of execution, it is more than likely that writing tests with asserts in callbacks is what you will need anyway.

My question is do I have to implement promises all the time for such situations to block the execution?
Notice that promises don't block the execution. They defer the execution of code that depends on the result, notice that you're still chaining callbacks on them.
I am new to Javascript and the async nature of it and any insight when to use promises and when not to would be useful.
Promises are useful wherever you have some code that might run asynchronously and needs to pass back an asynchronous result. Otherwise you would need to use callbacks, which are way more ugly than promises.

This is part of code contracts and representing preconditions (what holds before you execute), postconditions (what holds after you execute), and object invariants (what can not change). JavaScript does not have native support for this, but you can use third party libraries (Cerny.js, ecmaDebug, jsContract, or jscategory)
I think it depends on your coding style, is it EAFP(Easier to ask for forgiveness than permission) or LBYL(Look before you leap). Both are viable! In most compiled languages you would use LBYL. However in Python for example you would use EAFP.
Generally if you know you will fail you want to fail fast. If you like to use assertions to ensure code fails fast it is up to you.

Synchronous promise resolution (bluebird vs. jQuery)

I have developed a small lib for the Dynamics CRM REST/ODATA webservice (CrmRestKit). The lib dependes on jQuery and utilizes the promise-pattern, repectivly the promise-like-pattern of jQuery.
Now I like to port this lib to bluebird and remove the jQuery dependency. But I am facing a problem because bluebird does not support the synchronous resolution of promise-objects.
Some context information:
The API of the CrmRestKit excepts an optional parameter that defines if the web-service call should be performed in sync or async mode:
CrmRestKit.Create( 'Account', { Name: "foobar" }, false ).then( function ( data ) {
....
} );
When you pass "true" or omit the last parameter, will the method created the record in sync. mode.
Sometimes it is necessary to perform a operation in sync-mode, for instance you can write JavaScript code for Dynamics CRM that is involed for the save-event of an form and in this event-handler you need to perform sync-operation for validation (e.g. validate that a certain number of child-records exist, in case the right number of records exist, cancel the save-operation and show an error message).
My problem now is the following: bluebird does not support the resolution in sync-mode. For instance when I do the following, the "then" handler is invoked in async fashion:
function print( text ){
console.log( 'print -> %s', text );
return text;
}
///
/// 'Promise.cast' cast the given value to a trusted promise.
///
function getSomeTextSimpleCast( opt_text ){
var text = opt_text || 'Some fancy text-value';
return Promise.cast( text );
}
getSomeTextSimpleCast('first').then(print);
print('second');
The output is the following:
print -> second
print -> first
I would expect that the "second" appears after the "first" because the promise is already resolved with an value. So I would assume that an then-event-handler is immediately invoked when applied on an already resolved promise-object.
When I do the same (use then on an already resolved promise) with jQuery I will have my expected result:
function jQueryResolved( opt_text ){
var text = opt_text || 'jQuery-Test Value',
dfd = new $.Deferred();
dfd.resolve(text);
// return an already resolved promise
return dfd.promise();
}
jQueryResolved('third').then(print);
print('fourth');
This will generate the following output:
print -> third
print -> fourth
Is there a way to make bluebird work in the same fashion?
Update:
The provided code was just to illustrate the problem. The idea of the lib is: Regardless of the execution-mode (sync, async) the caller will always deal with an promise-object.
Regarding "... asking the user... doesn't seems to make any sense": When you provide two methods "CreateAsync" and "CreateSync" it is also up to the user to decide how the operation is executed.
Anyway with the current implementation the default behavior (last parameter is optional) is a async execution. So 99% of the code requires a promise-object, the optional parameter is only use for the 1% cases where you simply need a sync execution. Furthermore I developed to lib for myself and I use in 99,9999% of the case the async mode but I thought it is nice to have the option to go the sync-road as you like.
But I thinks I got the point an sync method should simply return the value. For the next release (3.0) I will implement "CreateSync" and "CreateAsync".
Thanks for your input.
Update-2
My intension for the optional parameter was to ensure a consistend behavior AND prevent logic error. Assume your as a consumer of my methode "GetCurrentUserRoles" that uses lib. So the method will alway return an promise, that means you have to use the "then" method to execute code that depends on the result. So when some writes code like this, I agree it is totally wrong:
var currentUserRoels = null;
GetCurrentUserRoles().then(function(roles){
currentUserRoels = roles;
});
if( currentUserRoels.indexOf('foobar') === -1 ){
// ...
}
I agree that this code will break when the method "GetCurrentUserRoles" changes from sync to async.
But I understand that this I not a good design, because the consumer should now that he deals with an async method.

Short version: I get why you want to do that, but the answer is no.
I think the underlying question being asked is whether a completed promise should immediately run a callback, if the promise has already completed. I can think of a lot of reasons that this might happen - for example, an asynchronous save procedure that only saves data if changes were made. It may be able to detect changes from the client side in a synchronous fashion without having to go through an external resource, but if changes are detected then and only then would an asynchronous operation be required.
In other environments that have asynchronous calls, the pattern seems to be that the developer is responsible for understanding that their work might complete immediately (for example, .NET framework's implementation of the async pattern accomodates this). This is not a design problem of the framework, it's the way it's implemented.
JavaScript's developers (and many of the commenters above) seem to have a different point of view on this, insisting that if something might be asynchronous, it must always be asynchronous. Whether this is "right" or not is immaterial - according to the specification I found at https://promisesaplus.com/, item 2.2.4 states that basically no callbacks can be called until you are out of what I'll refer to as "script code" or "user code"; that is, the specification says clearly that even if the promise is completed you can't invoke the callback immediately. I've checked a few other places and they either say nothing on the topic or agree with the original source. I don't know if https://promisesaplus.com/ could be considered a definitive source of information in this regard, but no other sources that I saw disagreed with it and it seems to be the most complete.
This limitation is somewhat arbitrary and I frankly prefer the .NET perspective on this one. I'll leave it up to others to decide if they consider it "bad code" to do something that might or might not be synchronous in a way that looks asynchronous.
Your actual question is whether or not Bluebird can be configured to do the non-JavaScript behavior. Performance-wise there may be a minor benefit to doing so, and in JavaScript anything's possible if you try hard enough, but as the Promise object becomes more ubiquitous across platforms you will see a shift to using it as a native component instead of custom written polyfills or libraries. As such, whatever the answer is today, reworking a promise in Bluebird is likely to cause you problems in the future, and your code should probably not be written to depend on or provide immediate resolution of a promise.

You might think this is a problem, because there's no way to have
getSomeText('first').then(print);
print('second');
and to have getSomeText "first" printed before "second" when the resolution is synchronous.
But I think you have a logic problem.
If your getSomeText function may be synchronous or asynchronous, depending on the context, then it shouldn't impact the order of execution. You use promises to ensure it's always the same. Having a variable order of execution would likely become a bug in your application.
Use
getSomeText('first') // may be synchronous using cast or asynchronous with ajax
.then(print)
.then(function(){ print('second') });
In both cases (synchronous with cast or asynchronous resolution), you'll have the correct execution order.
Note that having a function being sometimes synchronous and sometimes not isn't a weird or unlikely case (think about cache handling, or pooling). You just have to suppose it's asynchronous, and all will be always fine.
But asking the user of the API to precise with a boolean argument if he wants the operation to be asynchronous doesn't seem to make any sense if you don't leave the realm of JavaScript (i.e. if you don't use some native code).

The point of promises is to make asynchronous code easier, i.e. closer to what you feel when using synchronous code.
You're using synchronous code. Don't make it more complicated.
function print( text ){
console.log( 'print -> %s', text );
return text;
}
function getSomeTextSimpleCast( opt_text ){
var text = opt_text || 'Some fancy text-value';
return text;
}
print(getSomeTextSimpleCast('first'));
print('second');
And that should be the end of it.
If you want to keep the same asynchronous interface even though your code is synchronous, then you have to do it all the way.
getSomeTextSimpleCast('first')
.then(print)
.then(function() { print('second'); });
then gets your code out of the normal execution flow, because it's supposed to be asynchronous. Bluebird does it the right way there. A simple explanation of what it does:
function then(fn) {
setTimeout(fn, 0);
}
Note that bluebird doesn't really do that, it's just to give you a simple example.
Try it!
then(function() {
console.log('first');
});
console.log('second');
This will output the following:
second
first

There are some good answers here already, but to sum up the crux of the matter very succinctly:
Having a promise (or other async API) that is sometimes asynchronous and sometimes synchronous is a bad thing.
You may think it's fine because the initial call to your API takes a boolean to switch off between sync/async. But what if that's buried in some wrapper code and the person using that code doesn't know about these shenanigans? They've just wound up with some unpreditable behavior through no fault of their own.
The bottom line: Don't try to do this. If you want synchronous behavior, don't return a promise.
With that, I'll leave you with this quotation from You Don't Know JS:
Another trust issue is being called "too early." In application-specific terms, this may actually involve being called before some critical task is complete. But more generally, the problem is evident in utilities that can either invoke the callback you provide now (synchronously), or later (asynchronously).
This nondeterminism around the sync-or-async behavior is almost always going to lead to very difficult to track down bugs. In some circles, the fictional insanity-inducing monster named Zalgo is used to describe the sync/async nightmares. "Don't release Zalgo!" is a common cry, and it leads to very sound advice: always invoke callbacks asynchronously, even if that's "right away" on the next turn of the event loop, so that all callbacks are predictably async.
Note: For more information on Zalgo, see Oren Golan's "Don't Release Zalgo!" (https://github.com/oren/oren.github.io/blob/master/posts/zalgo.md) and Isaac Z. Schlueter's "Designing APIs for Asynchrony" (http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony).
Consider:
function result(data) {
console.log( a );
}
var a = 0;
ajax( "..pre-cached-url..", result );
a++;`
Will this code print 0 (sync callback invocation) or 1 (async callback invocation)? Depends... on the conditions.
You can see just how quickly the unpredictability of Zalgo can threaten any JS program. So the silly-sounding "never release Zalgo" is actually incredibly common and solid advice. Always be asyncing.

What about this case, also CrmFetchKit related which in latest version uses Bluebird. I have upgraded from version 1.9 that was based on jQuery. Still the old app code that uses CrmFetchKit has methods the prototypes of which I can't or won't change.
Existing App Code
CrmFetchKit.FetchWithPaginationSortingFiltering(query.join('')).then(
function (results, totalRecordCount) {
queryResult = results;
opportunities.TotalRecords = totalRecordCount;
done();
},
function err(e) {
done.fail(e);
}
);
Old CrmFetchKit implementation (a custom version of fetch())
function fetchWithPaginationSortingFiltering(fetchxml) {
var performanceIndicator_StartTime = new Date();
var dfd = $.Deferred();
fetchMore(fetchxml, true)
.then(function (result) {
LogTimeIfNeeded(performanceIndicator_StartTime, fetchxml);
dfd.resolve(result.entities, result.totalRecordCount);
})
.fail(dfd.reject);
return dfd.promise();
}
New CrmFetchKit implementation
function fetch(fetchxml) {
return fetchMore(fetchxml).then(function (result) {
return result.entities;
});
}
My problem is that the old version had the dfd.resolve(...) where I was able to pass any number of params that I need.
The new implementation just returns, the parent seems to call the callback, I can't call it directly.
I went and made a custom version of the fetch() in the new implementation
function fetchWithPaginationSortingFiltering(fetchxml) {
var thePromise = fetchMore(fetchxml).then(function (result) {
thePromise._fulfillmentHandler0(result.entities, result.totalRecordCount);
return thePromise.cancel();
//thePromise.throw();
});
return thePromise;
}
But the problem is that the callback gets called two times, once when I do it explicitly and second by the framework but it passes it one parameter only. To trick it and "tell" not to call anything because I do it explicitly I try to call .cancel() but it is ignored. I understood why but still how do you do the "dfd.resolve(result.entities, result.totalRecordCount);" in the new version with out having to changes prototypes in the app that uses this library ?

You can in fact do this, yes.
Modify the bluebird.js file (for npm: node_modules/bluebird/js/release/bluebird.js), with the following change:
[...]
target._attachExtraTrace(value);
handler = didReject;
}
- async.invoke(settler, target, {
+ settler.call(target, {
handler: domain === null ? handler
: (typeof handler === "function" &&
[...]
For more info, see here: https://github.com/stacktracejs/stacktrace.js/issues/188

JQuery blocking for an asynch initialization

I'm writing an AngularJS service for a SignalR hub. Here's my factory for the service:
.factory('gameManager', [function () {
$.connection.hub.start();
var manager = $.connection.gameManager;
return manager;
}])
That code would be perfect, except that that .start() call is asynchronous, and the hub has not completed starting by the time the manager is returned. Basically, I want to block until the start is complete before returning the manager. The .start() method returns a Jquery deferred object, which I'm guessing is part of the answer, but I don't know how to use it without a callback function?

Something like the following should do the trick.
app.factory('gameManager', [function () {
return $.connection.hub.start().then(function() {
return $.connection.gameManager;
});
}])
Now your callback function will return a deferred/promise too, so the service consumer will need to be expecting that. Your consuming code might look something like this:
gameManager.then(function(gameManager) {
// do whatever with game manager
gameManager.doSomething();
});
The docs for jquery Deferred are here. In particular, check out Deferred.then().
Note that:
the deferred.then() method returns a new promise that can filter the status and values of a deferred through a function ... These filter functions can return a new value to be passed along to the promise's .done() or .fail() callbacks, or they can return another observable object (Deferred, Promise, etc) which will pass its resolved / rejected status and values to the promise's callbacks...
update:
An alternate approach (and probably the better approach - since it won't require that your consumer handle the promise) is to let the hub initialize completely, before setting up your factory, and kicking off your app controller. Something like this...
$.connection.hub.start().then(function() {
app.factory('gameManager', function() {
return $.connection.gameManager;
});
// ...
// kick off the rest of the app..
});

You will not find what you are looking for, you will have to go with Lee's answer. Javascript is mostly single-threaded and does not allow blocking (with specific exceptions, such as alert window or synchronous ajax call).