Develop Reference

Nester deferred ajax calls in loops

I've got a complicated (at least for me) set up of nested loops, ajax calls, and deferreds. The code is calling an API, parsing out relevant data, then using it to make further calls to other APIs.
It works almost as intended. I used the answer to this question (Using $.Deferred() with nested ajax calls in a loop) to build it. Here's my code:
function a() {
var def = $.Deferred();
var req = [];
for (var i = 0 /*...*/) {
for (var j = 0 /*...*/) {
(function(i, j) {
req.push($.ajax({
//params
}).done(function(resp) {
var def2 = $.Deferred();
var req2 = [];
for (var k = 0 /*...*/) {
for (var l = 0 /*...*/) {
req2.push(b(l));
}
}
$.when.apply($, req2).done(function() {
console.log("Got all data pieces");
def2.resolve();
})
}));
})(i, j);
}
}
$.when.apply($, req).done(function() {
console.log("Got all data");
def.resolve();
});
return def.promise();
}
function b(j) {
var def = $.Deferred();
$.when.apply(
$.ajax({
//params
})
).then(function() {
console.log("Got data piece #" + l);
def.resolve();
});
return def.promise();
}
function main() {
//...
$.when.apply($, a()).then(function() {
console.log("All done");
displayPage();
})
//...
}
Here's what I'm expecting to see when the calls complete
(In no specific order)
Got data piece #1
Got data piece #0
Got data piece #2
Got all data pieces
Got data piece #2
Got data piece #1
Got data piece #0
Got all data pieces
Got data piece #0
Got data piece #1
Got data piece #2
Got all data pieces
Got all data <-- These two must be last, and in this order
All done
Here's what I'm seeing
All done
Got data piece #0
Got data piece #1
Got data piece #2
Got all data pieces
Got data piece #0
Got data piece #1
Got data piece #2
Got all data pieces
Got data piece #0
Got data piece #1
Got data piece #2
Got all data pieces
I stepped through it in the debugger, and the 'Got all data' line in function a() gets printed in the correct sequence after everything else completes, after which def.resolve() should get called and resolve the returned promise.
However, in main(), a() is seen as resolved right away and the code jumps right into printing 'All done' and displaying the page. Any ideas as to why it doesn't wait as it's supposed to?

You have illustrated a set of code and said it isn't doing what you expected, but you haven't really described the overall problem. So, I don't actually know exactly what code to recommend. We do a lot better here with real problems rather than pseudo code problems. So, instead, what I can do is to outline a bunch of things that are wrong with your code:
Expecting serial order of parallel async operations
Based on what you say you are expecting, the basic logic for how you control your async operations seems to be missing. When you use $.when() on a series of promises that have already been started, you are running a whole bunch of async operations in parallel. Their completion order is completely unpredictable.
Yes, you seem to expect to be able to run a whole bunch of b(i) in parallel and have them all complete in order. That seems to be the case because you say you are expecting this type of output:
Got data piece #0
Got data piece #1
Got data piece #2
where each of those statements is generated by the completion of some b(i) operation.
That simply will not happen (or it would be blind luck if it did in the real world because there is no code that guarantees the order). Now, you can run them in parallel and use $.when() to track them and $.when() will let you know when they are all done and will collect all the results in order. But when each individual async operation in that group finishes is up to chance.
So, if you really wanted each of your b(i) operations to run and complete in order, then you would have to purposely sequence them (run one, wait for it to complete, then run the next, etc...). In general, if one operation does not depend upon the other, it is better to run them in parallel and let $.when() track them all and order the results for you (because you usually get your end result faster by running them all in parallel rather than sequencing them).
Creation of unnecessary deferreds in lots of places - promse anti-pattern
In this code, there is no need to create a deferred at all. $.ajax() already returns a promise. You can just use that promise. So, instead of this:
function b(j) {
var def = $.Deferred();
$.when.apply(
$.ajax({
//params
})
).then(function() {
console.log("Got data piece #" + l);
def.resolve();
});
return def.promise();
}
You can do this:
function b(j) {
return $.ajax({
//params
}).then(function(data) {
console.log("Got data piece #" + l);
return data;
});
}
Note, that you just directly return the promise that is already produced by $.ajax() and no deferred needs to be created at all. This is also a lot more bulletproof for error handling. One of the reason your method is called an anti-pattern is you don't handle errors at all (a common mistake when using this anti-pattern). But, the improved code, propagates errors right back to the caller just like they should be. In your version, if the $.ajax() call rejects its promise (due to an error), your deferred is NEVER resolved and the caller never sees the error either. Now, you could write extra code to handle the error, but there is no reason to. Just return the promise you already have. When coding with async operations that return promises, you should pretty much never need to create your own deferred.
$.when() is only needed when you have more than one promise
In your b() function, there is no need to use $.when() in this piece of code:
$.when(
$.ajax({
//params
})).then(...);
When you have a single promise, you just use .then() directly on it.
$.ajax({
//params
}).then(...);
Only use $.when() when you have more than one promise and you want to know when all of them are done. If you only have one promise, just use its own .then() handler.
More anti-pattern - just return promises from .then() handlers
In your inner loop, you have this:
$.when.apply($, req2).done(function() {
console.log("Got all data pieces");
def2.resolve();
})
There are several things wrong here. It's not clear what you're trying to do because def2 is a deferred that nothing else uses. So, it appears you're trying to tell someone when this req2 group of promises is done, but nobody is using it. In addition it's another version of the anti-pattern. $.when() already returns a promise. You don't need to create a deferred to resolve when $.when() completes. You can just use the promise that $.when() already returns.
Though I don't fully know your intent here, it appears that what you should probably do is to get rid of the def2 deferred entirely and do just this:
return $.when.apply($, req2).done(function() {
console.log("Got all data pieces");
});
Returning this promise from the .then() handler that it is within will chain this sequence of actions to the parent promise and make the parent promise wait for this new promise to be resolved (which is tied to when all the req2 promises are done) before the parent promise will resolve. This is how you make parent promises dependent upon other promise within a .then() handler. You return a promise from the .then() handler.
And, the exact same issue is true for your outer $.when.apply($, req) also. You don't need a deferred there at all. Just use the promise that $.when() already returns.
Putting it together
Here's a cleaned up version of your code that gets rid of the anti-patterns in multiple places. This does not change the sequencing of the b(i) calls among themselves. If you care about that, it is a bigger change and we need to see more of the real/actual problem to know what best to recommend.
function a() {
var req = [];
for (var i = 0 /*...*/) {
for (var j = 0 /*...*/) {
(function(i, j) {
req.push($.ajax({
//params
}).then(function(resp) {
var req2 = [];
for (var k = 0 /*...*/) {
for (var l = 0 /*...*/) {
req2.push(b(l));
}
}
return $.when.apply($, req2).done(function() {
console.log("Got all data pieces");
});
}));
})(i, j);
}
}
return $.when.apply($, req).done(function() {
console.log("Got all data");
});
}
function b(j) {
return $.ajax({
//params
}).then(function(data) {
console.log("Got data piece #" + l);
return data;
});
}
function main() {
//...
a().then(function() {
console.log("All done");
displayPage();
});
//...
}
P.S. If you want to process the b(i) results from within the same group in order, then don't use a .then() handler on the individual promise because those will execute in arbitrary order. Instead, use the results that come with $.when().then(result1, result2, ...) and process them all there. Though the individual promises complete in an arbitrary order, $.when() will collect the results into the original order so if you process the results in the $.when() handler, then you can process them all in order.

Order of promises in AngularJS

Question:
Is there an "easy" way to cancel ($q-/$http-)promises in AngularJS or determine the order in which promises were resolved?
Example
I have a long running calculation and i request the result via $http. Some actions or events require me to restart the calculation (and thus sending a new $http request) before the initial promise is resolved. Thus i imagine i can't use a simple implementation like
$http.post().then(function(){
//apply data to view
})
because I can't ensure that the responses come back in the order in which i did send the requests - after all i want to show the result of the latest calculation when all promises were resolved properly.
However I would like to avoid waiting for the first response until i send a new request like this:
const timeExpensiveCalculation = function(){
return $http.post().then(function(response){
if (isNewCalculationChained) {return timeExpensiveCalculation();}
else {return response.data;}
})
}
Thoughts:
When using $http i can access the config-object on the response to use some timestamps or other identifiers to manually order the incoming responses. However i was hoping I could just tell angular somehow to cancel an outdated promise and thus not run the .then() function when it gets resolved.
This does not work without manual implementation for $q-promises instead of $http though.
Maybe just rejecting the promise right away is the way to go? But in both cases it might take forever until finally a promise is resolved before the next request is generated (which leads to an empty view in the meantime).
Is there some angular API-Function that i am missing or are there robust design patterns or "tricks" with promise chaining or $q.all to handle multiple promises that return the "same" data?

I do it by generating a requestId, and in the promise's then() function I check if the response is coming from the most recent requestId.
While this approach does not actually cancel the previous promises, it does provide a quick and easy way to ensure that you are handling the most recent request's response.
Something like:
var activeRequest;
function doRequest(params){
// requestId is the id for the request being made in this function call
var requestId = angular.toJson(params); // I usually md5 hash this
// activeRequest will always be the last requestId sent out
activeRequest = requestId;
$http.get('/api/something', {data: params})
.then(function(res){
if(activeRequest == requestId){
// this is the response for last request
// activeRequest is now handled, so clear it out
activeRequest = undefined;
}
else {
// response from previous request (typically gets ignored)
}
});
}
Edit:
On a side-note, I wanted to add that this concept of tracking requestId's can also be applied to preventing duplicate requests. For example, in my Data service's load(module, id) method, I do a little process like this:
generate the requestId based on the URL + parameters.
check in requests hash-table for the requestId
if requestId is not found: generate new request and store promise in hash-table
if requestId is found: simply return the promise from the hash-table
When the request finishes, remove the requestId's entry from the hash-table.

Cancelling a promise is just making it not invoke the onFulfilled and onRejected functions at the then stage. So as #user2263572 mentioned it's always best to let go the promise not cancelled (ES6 native promises can not be cancelled anyways) and handle this condition within it's then stage (like disregarding the task if a global variable is set to 2 as shown in the following snippet) and i am sure you can find tons of other ways to do it. One example could be;
Sorry that i use v (looks like check character) for resolve and x (obvious) for reject functions.
var prom1 = new Promise((v,x) => setTimeout(v.bind(null,"You shall not read this"),2000)),
prom2,
validPromise = 1;
prom1.then(val => validPromise === 1 && console.log(val));
// oh what have i done..!?! Now i have to fire a new promise
prom2 = new Promise((v,x) => setTimeout(v.bind(null,"This is what you will see"),3000));
validPromise = 2;
prom2.then(val => validPromise === 2 && console.log(val));

I'm still trying to figure out a good way to unit test this, but you could try out this kind of strategy:
var canceller = $q.defer();
service.sendCalculationRequest = function () {
canceller.resolve();
return $http({
method: 'GET',
url: '/do-calculation',
timeout: canceller.promise
});
};

In ECMA6 promises, there is a Promise.race(promiseArray) method. This takes an array of promises as its argument, and returns a single promise. The first promise to resolve in the array will hand off its resolved value to the .then of the returned promise, while the other array promises that came in second, etc., will not be waited upon.
Example:
var httpCall1 = $http.get('/api/something', {data: params})
.then(function(val) {
return {
id: "httpCall1"
val: val
}
})
var httpCall2 = $http.get('/api/something-else', {data: params})
.then(function(val) {
return {
id: "httpCall2"
val: val
}
})
// Might want to make a reusable function out of the above two, if you use this in Production
Promise.race([httpCall1, httpCall2])
.then(function(winningPromise) {
console.log('And the winner is ' + winningPromise.id);
doSomethingWith(winningPromise.val);
});
You could either use this with a Promise polyfil, or look into the q.race that someone's developed for Angular (though I haven't tested it).

Using promises/callbacks to wait for function to finish in javascript

I have a function which creates a database object out of three arrays. The arrays are filled in an each loop, one of the arrays relies on the value in the same iteration of the loop.
The dependent array uses the requests library and the cheerio library to grab a string to populate the array with.
Currently the dependent array fills with nulls which I think is because the loop is not waiting for the request to be returned.
I am still learning and would like to get this to work without direct blocking to keep things asynchronous so I'm looking into promises/callbacks.
This is being done server-side but from what I've seen in cheerios docs there is no promises capability.
Here's what I have so far. (getFile() is the function that isn't filling the 'c' array, it also depends on the current value being put into 'b'). I do know that the getFile function gets the correct value with a console log test, so the issue must be in the implementation of filling 'c'.
addToDB() is a function which saves a value into mongoDB, from testing I know that the objects are correctly being put into the db, just the c array is not correct.
function getInfo(path) {
$(path).each(function(i,e) {
a.push(...)
b.push(value)
c.push(getFile(value))
})
var entry = new DB...//(a,b,c)
addToDB(entry);
}
function getFile(...) {
request(fullUrl, function (err, resp, page) {
if (!err && resp.statusCode == 200) {
var $ = cheerio.load(page); // load the page
srcEp = $(this).attr("src");
return srcEp;
} // end error and status code
}); // end request
}
I've been reading about promises/callbacks and then() but I've yet to find anything which works.

First, you have to get your mind around the fact that any process that relies, at least in part, on an asynchronous sub-process, is itself inherently asynchronous.
At the lowest level of this question's code, request() is asynchronous, therefore its caller, getFile() is asynchronous, and its caller, getInfo() is also asynchronous.
Promises are an abstraction of the outcome of asynchronous processes and help enormously in coding the actions to be taken when such processes complete - successfully or under failure.
Typically, low-level asynchronous functions should return a promise to be acted on by their callers, which will, in turn, return a promise to their callers, and so on up the call stack. Inside each function, returned promise(s) may be acted on using promise methods, chiefly .then(), and may be aggregated using Promise.all() for example (syntax varies).
In this question, there is no evidence that request() currently returns a promise. You have three options :
discover that request() does, in fact, return a promise.
rewrite request() to return a promise.
write an adapter function (a "promisifier") that calls request(), and generates/returns the promise, which is later fulfilled or rejected depending on the outcome of request().
The first or second options would be ideal but the safe assumption for me (Roamer) is to assume that an adapter is required. Fortunately, I know enough from the question to be able to write one. Cheerio appears not to include jQuery's promise implementation, so a dedicated promise lib will be required.
Here is an adapter function, using syntax that will work with the Bluebird lib or native js promises:
//Promisifier for the low level function request()
function requestAsync(url) {
return new Promise(function(resolve, reject) {
request(url, function(err, resp, page) {
if (err) {
reject(err);
} else {
if (resp.statusCode !== 200) {
reject(new Error('request error: ' + resp.statusCode));
}
} else {
resolve(page);
}
});
});
}
Now getFile(...) and getInfo() can be written to make use of the promises returned from the lowest level's adapter.
//intermediate level function
function getFile(DOMelement) {
var fullUrl = ...;//something derived from DOMelement. Presumably either .val() or .text()
return requestAsync(fullUrl).then(function (page) {
var $ = cheerio.load(page);
srcEp = $(???).attr('src');//Not too sure what the selector should be. `$(this)` definitely won't work.
return srcEp;
});
}
//high level function
function getInfo(path) {
var a = [], b = [], c = [];//presumably
// Now map the $(path) to an array of promises by calling getFile() inside a .map() callback.
// By chaining .then() a/b/c are populated when the async data arrives.
var promises = $(path).map(function(i, e) {
return getFile(e).then(function(srcEp) {
a[i] = ...;
b[i] = e;
c[i] = srcEp;
});
});
//return an aggregated promise to getInfo's caller,
//in case it needs to take any action on settlement.
return Promise.all(promises).then(function() {
//What to do when all promises are fulfilled
var entry = new DB...//(a,b,c)
addToDB(entry);
}, function(error) {
//What to do if any of the promises fails
console.log(error);
//... you may want to do more.
});
}

How to get around the asynchronous Node.js behaviour?

I'm working on a script that pings websites and returns the results in a web UI. However, I've run into a problem which I am trying to figure out the best solution for.
This block of code needs to return a array of statuses but due to the asynchronous behaviour of Node.js, it returns an empty array because the code takes time to execute.
Here is what I have:
var ping = require('ping');
function checkConnection(hosts) {
var results = [];
hosts.forEach(function (host) {
ping.sys.probe(host, function (isAlive) {
results.push({"host": host, "status": isAlive});
});
});
return {results: results, timestamp: new Date().getTime()};
}
module.exports.checkConnection = checkConnection;
I know that you could solve this problem with the use of timers but what would be the simples and most ideal solution here?

How to get around the asynchronous Node.js behaviour?
Don't. Instead, embrace it, by having your checkConection accept a callback or return a promise.
Callback example:
function checkConnection(hosts, callback) {
var results = [];
hosts = hosts.slice(0); // Copy
hosts.forEach(function (host) {
ping.sys.probe(host, function (isAlive) {
results.push({"host": host, "status": isAlive});
if (results.length === hosts.length) {
callback({results: results, timestamp: new Date().getTime()});
}
});
});
}
Note the defensive shallow copy of hosts. If you don't do that, then since this code runs asynchronously, the calling code could add to or remove from the hosts array while you were processing responses, and the lengths would never match.
An alternate way to handle that without copying is to simply count how many requests you've initiated:
function checkConnection(hosts, callback) {
var results = [];
var requests = hosts.length;
hosts.forEach(function (host) {
ping.sys.probe(host, function (isAlive) {
results.push({"host": host, "status": isAlive});
if (results.length === requests) {
callback({results: results, timestamp: new Date().getTime()});
}
});
});
}
That looks like it sets up a race condition (what if something modifies hosts after you set requests but before you're done initiating your probe queries?) but it doesn't, because Node runs your JavaScript on a single thread, so no other code can reach in and modify hosts between the requests = hosts.length and hosts.forEach lines.

Like T.J. said, you will need to embrace asynchronous behavior if you are going to program in node.js as that is a fundamental tenet of how it works and how you code a responsive, scalable server using node.js.
T.J.'s answer is a straightforward way of solving this particular problem. But, since async issues will arise over and over again in node.js, promises can be a very useful tool for managing asynchronous behavior and they quickly become indispensable for more complicated multi-operation sequences with robust error handling.
So, here's a solution to your coding issue using Promises:
var ping = require('ping');
var Promise = require('bluebird');
// make a version of ping.sys.probe that returns a promise when done
ping.sys.probeAsync = function(host) {
return new Promise(function(resolve, reject) {
ping.sys.probe(host, function(isAlive) {
resolve({"host": host, "status": isAlive});
});
}
}
function checkConnection(hosts) {
var promises = hosts.map(function(host) {
return ping.sys.probeAsync(host);
});
return Promise.all(promises).then(function(results) {
return {results: results, timestamp: new Date().getTime()};
});
}
module.exports.checkConnection = checkConnection;
Sample Usage:
myModule.checkConnection(myArrayOfHosts).then(function(results) {
// results is the {results: results, timestamp: time} object
});
Step-by-step, here's how this works:
Load the Bluebird promise library.
Create a promisified version of ping.sys.probe called ping.sys.probeAsync that returns a promise that will be resolved when the underlying call is done.
Using .map() on your array, create an array of promises from calling ping.sys.probeAsync on each item in the array.
Using Promise.all(), create a new promise that is the aggregation of all the promises in the array. It will call it's .then() handler only when all the promises in the array have been resolved (e.g. have finished).
Add a .then() handler to Promise.all() so the timestamp can be added to the results.
Return the Promise.all() promise so the caller of checkConnection() gets a promise back they can use.
When calling checkConnection() use a .then() handler to know when all the operations are done and to obtain the results.
Hopefully you can see that once you have a promisified version of your function and you understand how promises work, you can then write the actual async code much simpler. And, if you also had error handling or had a sequence of async operations that had to be run one after the other (something you don't have here), the advantages of using promises is even greater.
P.S. I think Bluebird's Promise.map() can be used to combine the hosts.map() and Promise.all() into a single call, but I've not used that function myself so I didn't offer it here.

How can I add a new jquery deferred to an existing $.when?

I am refactoring a resource-loading function that used a traditional callback pattern to instead use jQuery Deferreds.
This function takes and array of urls, creates a new Deferred object for each resource, creates a $.when Deferred object to watch them, and returns the promise of the $.when object.
Here's a simplified version of the method:
theLib = {
getResources : function(paths) {
var deferreds = [];
theLib.isLoading = true;
$.each(paths, function(i, path) {
// do something with the path, either using getScript
// or making a new $.Deferred as needed
deferreds.push(/* the deferred object from above comment */);
});
theLib.currentDeferred = $.when.apply(null,deferreds).always(function() {
theLib.isLoading = false;
});
return theLib.currentDeferred.promise();
};
This works well.
My problem: In the old script, not only would one call theLib.getResources() based on user actions or events, but one would also define a master list of resources that the application will "stream" while the user is not taking any action (i.e. reading an article).
Some of these streamed resources would be the same resources that could be called manually when a user does take action. The script was smart enough not to load a resource twice by keeping track of what was loaded.
It also keeps track of theLib.isLoading. The beginning of that function looked something like this:
getResources : function(paths, callback) {
if (theLib.isLoading) {
settimeout(function() {
theLib.getResources(paths, callback);
}, 100);
}
theLib.isLoading = true;
I can't do this anymore, because I need to return a promise object.
I know that I can check theLib.currentDeferred.isResolved(). At that point, if it is not resolved: How can I add more deferred objects to the $.when queue that is being watched?

I guess I needed to ask the question to find a solution for myself. Basically I added the following code the beginning of getResources:
if ( ! theLib.currentDeferred.isResolved()) {
return $.when(theLib.currentDeferred).always(function() {
theLib.getResources(paths);
}).promise();
}
The above was failing. The correct solution was to pipe the results:
if ( ! theLib.currentDeferred.isResolved()) {
return theLib.currentDeferred.pipe(function() {
return theLib.getResources(paths);
});
}