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JavaScript Promises with AJAX

I am trying to write a series of AJAX requests into a dictionary.
I am attempting to use promises for this, however I am either writing the promise syntax incorrectly, or what I think may be happening is that the function is actually completing (for loop is done, and AJAX requests sent) but the AJAX requests are still not returned. Therefore this is still returning an empty dictionary.
let dict = {};
let activeMachines = ["41", "42", "43"];
let dataPromise = new Promise (function (resolve, reject)
{
for (let i = 0; i < activeMachines.length; i++)
{
let machineID = activeMachines[i]
let getAPIData = new XMLHttpRequest();
let url = 'http://127.0.0.1:8000/processes/apidata/' +machineID + '/';
getAPIData.open('GET', url);
getAPIData.send();
getAPIData.onload = function()
{
let APIData = JSON.parse(getAPIData.responseText);
dict['machine_' + machineID] = APIData[0].author_id;
dict['temp' + machineID] = APIData[0].tempData; //get value
dict['humid' + machineID] = APIData[0].humidData;
timeValue = String((APIData[0].dateTime));
dict['time' + machineID] = new Date(timeValue);
console.log("done");
}
}
resolve();
});
dataPromise.then(function() {console.log(dict);});
Is there a way to "sense" when all of the XMLHTTPRequests have returned?

#Rafael's answer will work, but it doesn't illuminate much about what's going wrong, since you're trying to grok the concept of Promises and write one yourself.
Fundamentally I think your approach has two missteps: 1. creating a single Promise that handles calls to all of your arbitrary list of "activeMachines", and 2. putting your resolve() call in the wrong place.
Usually a Promise looks like this:
const myPromise = new Promise(function(resolve, reject) {
doSomeAsyncWork(function(result) {
// Some kind of async call with a callback function or somesuch...
resolve(result);
});
}).then(data => {
// Do something with the final result
console.log(data);
});
You can simulate some kind of arbitrary asynchronous work with setTimeout():
const myPromise = new Promise(function(resolve, reject) {
// Resolve with "Done!" after 5 seconds
setTimeout(() => {
resolve("Done!");
}, 5000);
}).then(data => {
console.log(data); // "Done!"
});
However your original code puts the resolve() call in a weird place, and doesn't even pass it any data. It looks sorta equivalent to this:
const myPromise = new Promise(function(resolve, reject) {
// Resolve with "Done!" after 5 seconds
setTimeout(() => {
// Doing some work here instead of resolving...
}, 5000);
resolve();
}).then(data => {
console.log(data); // This would be "undefined"
});
Where you're doing a console.log("done"); in your original code is actually where you should be doing a resolve(someData);!
You're also trying to do side effect work inside of your Promise's async function stuff, which is really weird and contrary to how a Promise is supposed to work. The promise is supposed to go off and do its async work, and then resolve with the resulting data -- literally with the .then() chain.
Also, instead of doing multiple asynchronous calls inside of your Promise, you should generalize it so it is reusable and encapsulates only a single network request. That way you can fire off multiple asynchronous Promises, wait for them all to resolve, and then do something.
const activeMachines = ["41", "42", "43"];
// Make a reusable function that returns a single Promise
function fetchAPI(num) {
return new Promise(function(resolve, reject) {
const getAPIData = new XMLHttpRequest();
const url = "http://127.0.0.1:8000/processes/apidata/" + num + "/";
getAPIData.open("GET", url);
getAPIData.send();
getAPIData.onload = function() {
const APIData = JSON.parse(getAPIData.responseText);
const resolveData = {};
resolveData["machine_" + num] = APIData[0].author_id;
resolveData["temp" + num] = APIData[0].tempData; //get value
resolveData["humid" + num] = APIData[0].humidData;
timeValue = String(APIData[0].dateTime);
resolveData["time" + num] = new Date(timeValue);
resolve(resolveData);
};
});
}
// Promise.all() will resolve once all Promises in its array have also resolved
Promise.all(
activeMachines.map(ea => {
return fetchAPI(ea);
})
).then(data => {
// All of your network Promises have completed!
// The value of "data" here will be an array of all your network results
});
The fetch() API is great and you should learn to use that also -- but only once you understand the theory and practice behind how Promises actually operate. :)

Here's an example of the Fetch API which uses Promises by default:
let m_ids = [1,2,3,4];
let forks = m_ids.map(m => fetch(`http://127.0.0.1:8000/processes/apidata/${m}`));
let joined = Promise.all(forks);
joined
.then(files => console.log('all done', files))
.catch(error => console.error(error));
I hope this helps!

Issue with Nodejs promise

I have a sequence of function calls, connected with ES6 promises. Apparently, there is something wrong with this implementation, as API calls to the endpoint are not returning anything and the browser is stuck waiting for a response.
Please advise.
module.exports.insertTreatmentDetails = function (req, res) {
var doctorId = 10000
var departmentId = 10000
var procedureid = 10000
var hospitalSchema = new hospitalModel();
var p = new Promise(function (resolve, reject) {
counterSchema.getNext('Treatment.doctor.doctorId', collection, function (doctorId) {
doctorId = doctorId;
})
counterSchema.getNext('Treatment.departmentId', collection, function (departmentId) {
departmentId = departmentId
})
counterSchema.getNext('Treatment.procedureid', collection, function (procedureid) {
procedureid = procedureid
})
}).then(function () {
setData()
}).then(function (){
hospitalSchema.save(function (error, data) {
if (error) {
logger.error("Error while inserting record : - " + error)
return res.json({ "Message": error.message.split(":")[2].trim() });
}
else {
return res.json({ "Message": "Data got inserted successfully" });
}
});
});
};

The short answer is that you aren't calling resolve or reject inside the first promise in your chain. The promise remains in a pending state. Mozilla has a good basic explanation of promises.
How to Fix
It appears that you want to retrieve doctorId, departmentId, and procedureId before calling setData. You could try to wrap all three calls in one promise, checking whether all three have returned something in each callback, but the ideal is to have one promise per asynchronous task.
If it's feasible to alter counterSchema.getNext, you could have that function return a promise instead of accepting a callback. If not, I would recommend wrapping each call in its own promise. To keep most true to what your code currently looks like, that could look like this:
const doctorPromise = new Promise((resolve, reject) =>
counterSchema.getNext('Treatment.doctor.doctorId', collection, id => {
doctorId = id;
resolve();
}));
Then you could replace the first promise with a call to Promise.all:
var p = Promise.all([doctorPromise, departmentPromise, procedurePromise])
.then(setData)
.then(/* ... */);
Promises allow you to pass a value through to the next step, so if you wanted to get rid of your broadly-scoped variables (or set them in the same step where you call setData), you could just pass resolve as your callback to counterSchema.getNext and collect the values in the next step (also how you'd want to do it if you have counterSchema.getNext return a promise:
Promise.all([/* ... */])
.then(([doctorID, departmentID, procedureID]) => {
// If you aren't changing `setData`
doctorId = doctorID;
departmentId = departmentID;
procedureid = procedureID;
setData();
// If you are changing `setData`
setData(doctorID, departmentID, procedureID);
}).then(/* ... */).catch(/* I would recommend adding error handling */);

Method chaining and promises issue in Angular app with regards to returning a compared and updated array

I am trying to filter/match a list of returned IDs to a list of JSON data records, but I am struggling with (I believe) my promises and method chaining.
I can get the functions to work, except for when I add step 3 below. Then it resolves without the matching data (the function does carry on and eventually return the correct matching data, but by that time my method has already completed).
This is how it is supposed to work:
(getCompanyBrandProfileIDs) First my method gets a brandProfileID linked to the current user.
(getBrandProfiles) Then it takes the brandProfileID and get all brandProfiles linked to the specific brandProfile.
(getKeywordProfiles) Then it SHOULD take the returned brandProfiles, and get the matching keywordProfile for each brandProfile. It is an array of objects containing a brand_profile_id and and id.
This is my main method:
this.getCompanyBrandProfileIDs = function () {
var brandProfileIDsToReturn = $q.defer();
GetUserAccessService.returnBrandProfileID().then(function (brandProfileID) {
console.log(brandProfileID);
getBrandProfiles(brandProfileID).then(function (brandProfiles) {
console.log(JSON.stringify(brandProfiles));
var keywordProfilesArray = [];
getKeywordProfiles(brandProfiles).then(function (keywordProfiles) {
keywordProfilesArray = keywordProfiles;
console.log(JSON.stringify(keywordProfilesArray));
//brandProfileIDsToReturn.resolve(keywordProfilesArray);
});
brandProfileIDsToReturn.resolve(keywordProfilesArray);
});
});
return brandProfileIDsToReturn.promise;
};
This is the getBrandProfiles method:
function getBrandProfiles(brandProfileID) {
var getBrandProfilesLinkedToCompany = $q.defer();
pullSocialMediaData('keyword_profile_brand_profiles.json?brand_profile_id=' + brandProfileID).then(function (brandProfiles) {
var brandProfilesArray = [];
brandProfiles.forEach(function (profile) {
brandProfilesArray.push({ id: profile.id, name: profile.name });
});
getBrandProfilesLinkedToCompany.resolve(brandProfilesArray);
});
return getBrandProfilesLinkedToCompany.promise;
}
This is the getKeywordProfiles method:
function getKeywordProfiles(brandProfiles) {
var keywordProfilesToReturn = $q.defer();
var brandProfilesArray = brandProfiles;
var array = [];
brandProfilesArray.forEach(function (profile) {
findKeywordProfile(profile.id).then(function (keywordID) {
array.push(keywordID);
});
keywordProfilesToReturn.resolve(array);
})
return keywordProfilesToReturn.promise;
}
This is a helper method for getKeywordProfiles:
function findKeywordProfile(brandProfileID) {
var keywordProfileID = $q.defer();
pullSocialMediaData('list_keyword_profiles.json').then(function (data) {
var keywordProfileInstance = data.filter(function (keyword) {
return keyword.brand_profile_id === brandProfileID;
});
keywordProfileID.resolve(keywordProfileInstance[0].id);
});
return keywordProfileID.promise;
}
I would appreciate your assistance!

You are resolving the brandProfileIDsToReturn too soon. In this code: when you resolve the promise the then callback will not have been called, so keywordProfilesArray is guaranteed to be empty.
this.getCompanyBrandProfileIDs = function () {
var brandProfileIDsToReturn = $q.defer();
GetUserAccessService.returnBrandProfileID().then(function (brandProfileID) {
console.log(brandProfileID);
getBrandProfiles(brandProfileID).then(function (brandProfiles) {
console.log(JSON.stringify(brandProfiles));
var keywordProfilesArray = [];
getKeywordProfiles(brandProfiles).then(function (keywordProfiles) {
keywordProfilesArray = keywordProfiles;
console.log(JSON.stringify(keywordProfilesArray));
//brandProfileIDsToReturn.resolve(keywordProfilesArray);
});
brandProfileIDsToReturn.resolve(keywordProfilesArray);
});
});
return brandProfileIDsToReturn.promise;
};
Simply moving the resolve() call inside the then callback should fix it and in fact you have that line commented out, so uncomment it and remove the other resolve:
this.getCompanyBrandProfileIDs = function () {
var brandProfileIDsToReturn = $q.defer();
GetUserAccessService.returnBrandProfileID().then(function (brandProfileID) {
console.log(brandProfileID);
getBrandProfiles(brandProfileID).then(function (brandProfiles) {
console.log(JSON.stringify(brandProfiles));
var keywordProfilesArray = [];
getKeywordProfiles(brandProfiles).then(function (keywordProfiles) {
keywordProfilesArray = keywordProfiles;
console.log(JSON.stringify(keywordProfilesArray));
brandProfileIDsToReturn.resolve(keywordProfilesArray);
});
});
});
return brandProfileIDsToReturn.promise;
};
However you can probably simplify the code a lot if you stop using $q.defer(). Your functions already return promises so just return the promises they use and stop trying to create additional promises. I think this is equivalent to the previous code except it returns the promises directly, and I removed the log messages, and that means the getKeywordProfiles call simplifies down to a callback that just calls the function so you can pass the function directly:
this.getCompanyBrandProfileIDs = function () {
return GetUserAccessService.returnBrandProfileID().then(function (brandProfileID) {
return getBrandProfiles(brandProfileID).then(getKeywordProfiles);
});
});
};
and then you can simplify it further by extracting the inner .then:
this.getCompanyBrandProfileIDs = function () {
return GetUserAccessService.returnBrandProfileID()
.then(getBrandProfiles)
.then(getKeywordProfiles);
};
The getKeywordProfiles() function also needs to avoid resolving its promise until all of the findKeywordProfile() calls have resolved. Return a promise for the array of promises and when they resolve the promise will complete to an array of values:
function getKeywordProfiles(brandProfilesArray) {
var array = [];
brandProfilesArray.forEach(function (profile) {
array.push(findKeywordProfile(profile.id));
})
return $q.all(array);
}
To clarify my comments about $q, there are some cases where you need to create a promise from scratch using it, but they're fairly uncommon. Anything that happens asynchronously in Angular already returns a promise, and the great thing about promises is that they chain together, so when you have one promise calling .then() or .catch() will return a new one. Also the .then() callback can either return a value which resolves the new promise, or can return a promise which will only resolve the new promise when it, itself resolves. So just keep chaining the .then() calls together and each will wait for the previous one to complete.
$q is still useful though: $q.all() if you want to wait for a bunch of promises to all resolve, $q.race() if you have a bunch of promises and only one needs to resolve, $q.resolve(value) can also be useful as sometimes you just want a promise that will resolve immediately so you can hang a chain of other async functions off it.
Also it is safe to keep a promise around even long after it has resolved and you can still call .then() on it: this is useful if you have asynchronous initialisation code and events that may or may not be triggered before the initialisation has completed. No need to do if(isInitialised) when you can just do initPromise.then(...)

In getKeywordProfiles function you need resolve it when array loop finished.
function getKeywordProfiles(brandProfiles) {
var keywordProfilesToReturn = $q.defer();
var brandProfilesArray = brandProfiles;
var array = [];
brandProfilesArray.forEach(function (profile) {
findKeywordProfile(profile.id).then(function (keywordID) {
array.push(keywordID);
});
//--
//keywordProfilesToReturn.resolve(array);
})
//++
keywordProfilesToReturn.resolve(array);
return keywordProfilesToReturn.promise;
}
Info: I think you need to create an profileIdArray push all brandProfileID and send to your findKeywordProfile function. It will be more useful.

Promises for promises that are yet to be created without using the deferred [anti]pattern

Problem 1: only one API request is allowed at a given time, so the real network requests are queued while there's one that has not been completed yet. An app can call the API level anytime and expecting a promise in return. When the API call is queued, the promise for the network request would be created at some point in the future - what to return to the app? That's how it can be solved with a deferred "proxy" promise:
var queue = [];
function callAPI (params) {
if (API_available) {
API_available = false;
return doRealNetRequest(params).then(function(data){
API_available = true;
continueRequests();
return data;
});
} else {
var deferred = Promise.defer();
function makeRequest() {
API_available = false;
doRealNetRequest(params).then(function(data) {
deferred.resolve(data);
API_available = true;
continueRequests();
}, deferred.reject);
}
queue.push(makeRequest);
return deferred.promise;
}
}
function continueRequests() {
if (queue.length) {
var makeRequest = queue.shift();
makeRequest();
}
}
Problem 2: some API calls are debounced so that the data to be sent is accumulated over time and then is sent in a batch when a timeout is reached. The app calling the API is expecting a promise in return.
var queue = null;
var timeout = 0;
function callAPI2(data) {
if (!queue) {
queue = {data: [], deferred: Promise.defer()};
}
queue.data.push(data);
clearTimeout(timeout);
timeout = setTimeout(processData, 10);
return queue.deferred.promise;
}
function processData() {
callAPI(queue.data).then(queue.deferred.resolve, queue.deferred.reject);
queue = null;
}
Since deferred is considered an anti-pattern, (see also When would someone need to create a deferred?), the question is - is it possible to achieve the same things without a deferred (or equivalent hacks like new Promise(function (resolve, reject) {outerVar = [resolve, reject]});), using the standard Promise API?

Promises for promises that are yet to be created
…are easy to build by chaining a then invocation with the callback that creates the promise to a promise represents the availability to create it in the future.
If you are making a promise for a promise, you should never use the deferred pattern. You should use deferreds or the Promise constructor if and only if there is something asynchronous that you want to wait for, and it does not already involve promises. In all other cases, you should compose multiple promises.
When you say
When the API call is queued, the promise for the network request would be created at some point in the future
then you should not create a deferred that you can later resolve with the promise once it is created (or worse, resolve it with the promises results once the promise settles), but rather you should get a promise for the point in the future at which the network reqest will be made. Basically you're going to write
return waitForEndOfQueue().then(makeNetworkRequest);
and of course we're going to need to mutate the queue respectively.
var queue_ready = Promise.resolve(true);
function callAPI(params) {
var result = queue_ready.then(function(API_available) {
return doRealNetRequest(params);
});
queue_ready = result.then(function() {
return true;
});
return result;
}
This has the additional benefit that you will need to explicitly deal with errors in the queue. Here, every call returns a rejected promise once one request failed (you'll probably want to change that) - in your original code, the queue just got stuck (and you probably didn't notice).
The second case is a bit more complicated, as it does involve a setTimeout call. This is an asynchronous primitive that we need to manually build a promise for - but only for the timeout, and nothing else. Again, we're going to get a promise for the timeout, and then simply chain our API call to that to get the promise that we want to return.
function TimeoutQueue(timeout) {
var data = [], timer = 0;
this.promise = new Promise(resolve => {
this.renew = () => {
clearTimeout(timer);
timer = setTimeout(resolve, timeout);
};
}).then(() => {
this.constructor(timeout); // re-initialise
return data;
});
this.add = (datum) => {
data.push(datum);
this.renew();
return this.promise;
};
}
var queue = new TimeoutQueue(10);
function callAPI2(data) {
return queue.add(data).then(callAPI);
}
You can see here a) how the debouncing logic is completely factored out of callAPI2 (which might not have been necessary but makes a nice point) and b) how the promise constructor only concerns itself with the timeout and nothing else. It doesn't even need to "leak" the resolve function like a deferred would, the only thing it makes available to the outside is that renew function which allows extending the timer.

When the API call is queued, the promise for the network request would
be created at some point in the future - what to return to the app?
Your first problem can be solved with promise chaining. You don't want to execute a given request until all prior requests have finished and you want to execute them serially in order. This is exactly the design pattern for promise chaining. You can solve that one like this:
var callAPI = (function() {
var p = Promise.resolve();
return function(params) {
// construct a promise that chains to prior callAPI promises
var returnP = p.then(function() {
return doRealNetRequest(params);
});
// make sure the promise we are chaining to does not abort chaining on errors
p = returnP.then(null, function(err) {
// handle rejection locally for purposes of continuing chaining
return;
});
// return the new promise
return returnP;
}
})();
In this solution, a new promise is actually created immediately with .then() so you can return that promise immediately - there is no need to create a promise in the future. The actual call to doRealNetRequest() is chained to this retrurned .then() promise by returning its value in the .then() handler. This works because, the callback we provide to .then() is not called until some time in the future when the prior promises in the chain have resolved, giving us an automatic trigger to execute the next one in the chain when the prior one finishes.
This implementation assumes that you want queued API calls to continue even after one returns an error. The extra few lines of code around the handle rejection comment are there to make sure the chain continues even where a prior promise rejects. Any rejection is returned back to the caller as expected.
Here's a solution to your second one (what you call debounce).
the question is - is it possible to achieve the same things without a
deferred (or equivalent hacks like new Promise(function (resolve,
reject) {outerVar = [resolve, reject]});), using the standard Promise
API?
As far as I know, the debouncer type of problem requires a little bit of a hack to expose the ability to trigger the resolve/reject callbacks somehow from outside the promise executor. It can be done a little cleaner than you propose by exposing a single function that is within the promise executor function rather than directly exposing the resolve and reject handlers.
This solution creates a closure to store private state that can be used to manage things from one call to callAPI2() to the next.
To allow code at an indeterminate time in the future to trigger the final resolution, this creates a local function within the promise executor function (which has access to the resolve and reject functions) and then shares that to the higher (but still private) scope so it can be called from outside the promise executor function, but not from outside of callAPI2.
var callAPI2 = (function() {
var p, timer, trigger, queue = [];
return function(data) {
if (!p) {
p = new Promise(function(resolve) {
// share completion function to a higher scope
trigger = function() {
resolve(queue);
// reinitialize for future calls
p = null;
queue = [];
}
}).then(callAPI);
}
// save data and reset timer
queue.push(data);
clearTimeout(timer);
setTimeout(trigger, 10);
return p;
}
})();

You can create a queue, which resolves promises in the order placed in queue
window.onload = function() {
(function(window) {
window.dfd = {};
that = window.dfd;
that.queue = queue;
function queue(message, speed, callback, done) {
if (!this.hasOwnProperty("_queue")) {
this._queue = [];
this.done = [];
this.res = [];
this.complete = false;
this.count = -1;
};
q = this._queue,
msgs = this.res;
var arr = Array.prototype.concat.apply([], arguments);
q.push(arr);
msgs.push(message);
var fn = function(m, s, cb, d) {
var j = this;
if (cb) {
j.callback = cb;
}
if (d) {
j.done.push([d, j._queue.length])
}
// alternatively `Promise.resolve(j)`, `j` : `dfd` object
// `Promise` constructor not necessary here,
// included to demonstrate asynchronous processing or
// returned results
return new Promise(function(resolve, reject) {
// do stuff
setTimeout(function() {
div.innerHTML += m + "<br>";
resolve(j)
}, s || 0)
})
// call `cb` here, interrupting queue
.then(cb ? j.callback.bind(j, j._queue.length) : j)
.then(function(el) {
console.log("queue.length:", q.length, "complete:", el.complete);
if (q.length > 1) {
q.splice(0, 1);
fn.apply(el, q[0]);
return el
} else {
el._queue = [];
console.log("queue.length:", el._queue.length
, "complete:", (el.complete = !el._queue.length));
always(promise(el), ["complete", msgs])
};
return el
});
return j
}
, promise = function(t) {
++t.count;
var len = t._queue.length,
pending = len + " pending";
return Promise.resolve(
len === 1
? fn.apply(t, t._queue[0]) && pending
: !(t.complete = len === 0) ? pending : t
)
}
, always = function(elem, args) {
if (args[0] === "start") {
console.log(elem, args[0]);
} else {
elem.then(function(_completeQueue) {
console.log(_completeQueue, args);
// call any `done` callbacks passed as parameter to `.queue()`
Promise.all(_completeQueue.done.map(function(d) {
return d[0].call(_completeQueue, d[1])
}))
.then(function() {
console.log(JSON.stringify(_completeQueue.res, null, 2))
})
})
}
};
always(promise(this), ["start", message, q.length]);
return window
};
}(window));
window
.dfd.queue("chain", 1000)
.dfd.queue("a", 1000)
.dfd.queue("b", 2000)
.dfd.queue("c", 2000, function callback(n) {
console.log("callback at queue index ", n, this);
return this
}, function done(n) {
console.log("all done callback attached at queue index " + n)
})
.dfd.queue("do", 2000)
.dfd.queue("other", 2000)
.dfd.queue("stuff", 2000);
for (var i = 0; i < 10; i++) {
window.dfd.queue(i, 1000)
};
window.dfd.queue.apply(window.dfd, ["test 1", 5000]);
window.dfd.queue(["test 2", 1000]);
var div = document.getElementsByTagName("div")[0];
var input = document.querySelector("input");
var button = document.querySelector("button");
button.onclick = function() {
window.dfd.queue(input.value, 0);
input.value = "";
}
}
<input type="text" />
<button>add message</button>
<br>
<div></div>

Q Promise chain and NodeJS callbacks

I think this is a really stupid question but I'm having a hard time wrapping my head around promises.
I'm using Q (for nodejs) to sync up a couple of async functions.
This works like a charm.
var first = function () {
var d = Q.defer();
fs.readdir(path,function(err,files){
if(err) console.log(err);
d.resolve(files);
});
return d.promise;
};
var second = function (files) {
var list = new Array;
files.forEach(function(value, index){
var d = Q.defer();
console.log('looking for item in db', value);
db.query(
'SELECT * FROM test WHERE local_name =? ', [value],{
local_name : String,
},
function(rows) {
if (typeof rows !== 'undefined' && rows.length > 0){
console.log('found item!', rows[0].local_name);
d.resolve(rows[0]);
} else {
var itemRequest = value;
getItemData(itemRequest);
}
}
);
list.push(d.promise);
});
return Q.all(list);
};
first()
.then(second)
.done(function(list){
res.send(list);
});
The problem I have is with this little function:
getItemData(itemRequest)
This function is filled with several of callbacks. The promise chain runs through the function just fine but ignores all the callbacks I use ( eg several XHR calls I make in the function).
A simplified version of the function looks like this (just to give you an idea):
function getItemData(itemRequest){
helper.xhrCall("call", function(response) {
var requestResponse = JSON.parse(response)
, requestInitialDetails = requestResponse.results[0];
downloadCache(requestInitialDetails,function(image) {
image = localImageDir+requestInitialDetails.image;
helper.xhrCall("call2", function(response) {
writeData(item,image,type, function(){
loadData(item);
});
});
} else {
writeData(item,image,type, function(){
loadData(item);
});
}
});
});
The xhr function I use looks like this:
xhrCall: function (url,callback) {
var request = require("request")
, colors = require('colors');
request({
url: url,
headers: {"Accept": "application/json"},
method: "GET"
}, function (error, response, body) {
if(!error){
callback(body);
}else{
console.log('Helper: XHR Error',error .red);
}
});
}
So my questions:
Can I leave the function unaltered and use the callbacks that are in place ánd the promise chain?
Or do I have to rewrite the function to use promises for the XHR?
And if so, How can I best write my promise chain? Should I reject the initial promise in the forEach?
Again, sorry if this is a really stupid question but I don't know what the right course of action is here.
Thanks!
[EDIT] Q.nfcall, I don't get it
So I've been looking into Q.nfcall which allows me to use node callbacks. Bu I just don't understand exacly how this works.
Could someone give a simple example how I would go about using it for a function with several async xhr calls?
I tried this but as you can see I don't really understand what I'm doing:
var second = Q.nfcall(second);
function second (files) {
[EDIT 2]
This is the final funcction in my getitemdata function callback chain. This function basically does the same as the function 'second' but I push the result directly and then return the promise. This works as stated, but without all the additional callback data, because it does not wait for the callbacks to return with any data.
function loadData(item) {
var d = Q.defer();
db.query(
'SELECT * FROM test WHERE local_name =? ', [item],{
local_name : String,
},
function(rows) {
if (typeof rows !== 'undefined' && rows.length > 0){
list.push(d.promise);
}
}
);
});
return Q.all(list);
};

Your answer is not really clear after your second edit.
First, on your orignal question, your getItemData has no influence on the promise chain.
You could change you the function's call signature and pass your deferred promise like so.
getItemData(itemRequest, d)
and pass this deferred promises all the way to your xhrCall and resolve there.
I would re-write your whole implementation and make sure all your functions return promises instead.
Many consider deferred promises as an anti-pattern. So I use use the Promise API defined in harmony (the next javascript)
After said that, I would re-implement your original code like so (I've not tested)
var Promise = Promise || require('es6-promise').Promise // a polyfill
;
function errHandler (err){
throw err
}
function makeQuery () {
var queryStr = 'SELECT * FROM test WHERE local_name =? '
, queryOpt = {local_name: String}
;
console.log('looking for item in db', value)
return new Promise(function(resolve, reject){
db.query(queryStr, [value], queryOpt, function(rows) {
if (typeof rows !== 'undefined' && rows.length > 0){
console.log('found item!', rows[0].local_name);
resolve(rows[0]);
} else {
// note that it returns a promise now.
getItemData(value).then(resolve).catch(errHandler)
}
})
})
}
function first () {
return new Promise(function(resolve, reject){
fs.readdir(path, function(err, files){
if (err) return reject(err)
resolve(files)
})
})
}
function second (files) {
return Promise.all(files.map(function(value){
return makeQuery(value)
});
}
first()
.then(second)
.then(res.send)
.catch(errHandler)
Note that there is no done method on the Promise API.
One down side of the new Promise API is error handling. Take a look at bluebird.
It is a robust promise library which is compatible with the new promise API and has many of the Q helper functions.

As far as I can tell, you need to return a promise from getItemData. Use Q.defer() as you do in second(), and resolve it when the callbacks complete with the data. You can then push that into list.
To save code, you can use Q.nfcall to immediately call a node-style-callback function, and return a promise instead. See the example in the API docs: https://github.com/kriskowal/q/wiki/API-Reference#qnfcallfunc-args