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With a Promise is there a mechanism to access the final return value in a .then() chain?

Question: With a Promise is there a mechanism to access the final return value in a .then() chain?
Background I need to run a series of checks/adjustments on an object. A few of the checks are asynchronous for example comparing data against stored data in mongodb. Because some checks are asynchronous I believe a Promise chain may be the correct option. However because the checks must be done in a specific order Promise.all() will not work. If a Promise chain is a possibility after all of the checks and adjustments have been made I'm not sure how to retrieve the object from the last .then() in the chain. Perhaps I'm approaching this problem with the wrong tool.
The following code is a simplified example. myObject passes through the chain of .then() statements but I'm not sure how to retrieve the final, updated object or if that's even possible.
function promisesPromises() {
return new Promise( function(resolve, reject) {
let x = {
name: 'super duper',
randomDataOne: 10000,
randomDataTwo: 5000
};
if (x) {
resolve(x);
} else {
reject('uh oh');
}
});
}
function firstAdjustment(myObject) {
myObject.randomDataOne += 1000;
return myObject
}
function secondAdjustment(myObject) {
myObject.randomDataTwo += 500;
return myObject;
}
promisesPromises()
.then(firstAdjustment)
.then(secondAdjustment);

I would give async/await a try to simplify things.
Note: async functions always return a promise. So in order to get to the data you need to await for the returned data inside another async function, or you need to use then(). I have given both examples in the code below.
function promisesPromises() {
return new Promise( function(resolve, reject) {
let x = {
name: 'super duper',
randomDataOne: 10000,
randomDataTwo: 5000
};
// Promise set to resolve after 5 seconds
if (x) {
setTimeout(() => {
resolve(x);
}, 5000)
} else {
reject('uh oh');
}
});
}
function firstAdjustment(myObject) {
myObject.randomDataOne += 1000;
return myObject
}
function secondAdjustment(myObject) {
myObject.randomDataTwo += 500;
return myObject;
}
const asyncFunction = async () => {
const myObject = await promisesPromises()
firstAdjustment(myObject)
secondAdjustment(myObject)
return myObject
}
// Example retrieving data with then()
asyncFunction().then(res => console.log("Example using then(): ", res))
// Example retrieving data inside async function
const anotherAsyncFunction = async () => {
const result = await asyncFunction()
console.log("Example using async(): ", result)
}
anotherAsyncFunction()
// Timer countdown to promise resolve
let count = 1
const timer = setInterval(() => {
console.log(count++)
if (count > 4) clearInterval(timer)
}, 1000)

Correctly passing parameters to setTimeout

I am working on a Node.js project, and I am having issues wrapping a Promise function with a setTimeout function.
My original Promise function:
I want to wrap this function in a call to setTimeout() but am having trouble passing in the Promise object. I need the Promise object and the data object available inside the setTimeout() function, but when I pass them into setTimeout() as parameters i still get the following error:
TypeError: Cannot read property 'then' of undefined
My code:
return Promise.props(data).then(function (data) {
data.companies = data.order && data.order.companies;
if (!data.companies) {
data.companies = {};
data.companies[data.company.id] = data.company;
}
if (data.order) {
if (data.order.contactentry) {
data.order.pointofcontact = data.order.contactentry + ' ' + phone(data.order.contactentryphone);
} else if (data.order.borrowername) {
data.order.pointofcontact = data.order.borrowername + ' ' + phone(data.order.borrowerphone);
} else if (data.order.lockboxcode) {
data.order.pointofcontact = 'Lockbox ' + data.order.lockboxcode
}
}
if (data.part && data.order && data.part.vendor) {
var oid = data.order && data.order.id;
var vid = data.part && data.part.vendor && data.part.vendor.id;
if (!oid || !vid) {
var e = new Error('Could not assemble vendor accept url, order id or part vendor id are missing')
log.error({
error: e,
data,
}, e.message);
throw e;
}
}
return data;
});
The data object is available inside the function when passed in as a parameter, but the Promise object is not.
How can I properly pass in the Promise object to make it available inside setTimeout()?

You're approaching this inside-out. When you want to incorporate non-promise async code (such as setTimeout) into your promise-based code, you should isolate the wrapped non-promise part instead of getting it mixed too deep in the rest of your promise code.
Promise wrapper for setTimeout:
function delay(ms) {
return new Promise(function (resolve) {
setTimeout(resolve, ms);
});
}
Then you can use it:
return Promise.props(data)
.then(function (result) {
return delay(1000).return(result);
});
Since you appear to be using Bluebird, you can also skip all the setTimeout stuff and use the built-in .delay method:
return Promise.props(data).delay(1000);
Note that either of the above will add an additional 1 second delay to the time it takes to resolve all of the promises in data. If your goal is to just make it so that the minimum total time is 1 second, then you can use my approach from this question:
return Promise.delay(1000).return(Promise.props(data));

I would forget about using the arguments/return value of setTimeout, and just make use of new Promise
Delaying before Promise.props is called:
return new Promise(function (resolve) {
window.setTimeout(function () {
resolve(Promise.props(data));
}, 1000);
});
Or, delaying the result of Promise.props:
return Promise.props(data).then(function (data) {
return new Promise(function (resolve) {
window.setTimeout(function () {
resolve(data);
}, 1000);
});
});
Edit: If you're using Bluebird it looks like they have delay operators built in:
http://bluebirdjs.com/docs/api/delay.html
http://bluebirdjs.com/docs/api/promise.delay.html

Pattern for dynamic Javascript promises

Inside a promise, I need to call and process an indeterminate number of asynch API responses after individually calling them either inside another promise, or after said promise, but before another so the order of execution is respected.
var promiseA = function() {
return new Promise(function(resolve, reject) {
// 1. Establish objects needed from one API endpoint
// 2. Call API endpoint for each object and parse
// 3. Only then continue to next promise
}
}
var finalPromise = function() {
return new Promise(function(resolve, reject) {
//
}
}
promiseA()
.then(finalPromise)
So inside promiseA, I find out how many objects I'll need to poll individually from an API. Each request is of course asynchronous. I need to make these calls and process the response before the final promise is called.
I am struggling to determine a pattern for this with promises, where I can dynamically create these promises and only allow the final promise to execute after the indeterminate and asynchronous have executed and processed. I've worked with other languages where this is possible, but I'm struggling to see it here with Promises.
Any help is appreciated.

I have changed the answer to incorporate the comments below. Since, you mentioned ES6 promises I shall stick to that. There are two basic types of callbacks that we might care about.
DOM load or other one time event callbacks (window.onload and so on)
Async method callback (AJAX call, setTimout and so on)
Since,
1.DOM load or other one time event
var p = new Promise(function(res, rej) {
window.onload = res();
};
2.Plain callback: these are callbacks that don't conform to a convention. e.g. setTimeout
var p = new Promise(function(res, rej){
setTimeout(function() {
//your business/view logic
success? res():rej(); //if successful resolve else reject
}, 2000);
});
In each of the above case the promise (var p) can be wrapped to be returned by a function.
var myAsyncMethod = function () {
var p = new ... // as mentioned in 1 or 2
return p;
}
Then the usage:
myAsyncMethod()
.then(function(){/* success-handler */})
.catch(function(/* failure-handler */));
Specific to your question you may have many such methods:
function baseAJAXCall (url) {
new Promise(functoin(rej, res) {
$.get(url, function(err, data){
if(err) {
rej();
}
else {
resolve(data);
}
});
}
};
function callAPIEndpoint(url) {
return baseAJAXCall(url);
}
function finalPromiseHandler () {
//your final business/view logic
}
//USAGE
callAPIEndpoint('/my-first-call')
.then(function(data){
var promiseArray = data.map(function(item){
return baseAJAXCall(item.url);
});
return Promise.all(promiseArray);
})
.then(finalPromiseHandler)
.catch(function(){
console.log('.error-message.');
});
Ref:
How do I convert an existing callback API to promises?.
http://www.datchley.name/es6-promises/
Links from comments below.
---OLD ANSWER: PLEASE OVERLOOK---
I am familiar with this library : https://github.com/kriskowal/q. And, you can do this using using the q.all and q.allSettled constructs. May be that is what you are looking for.
Normally, the pattern is to create a function that returns a promise.
function someAsyncFuncName1(url) {
var def = q.defer();
//async function
$.get(url, function(err, data){ //suppose
if(err){
def.reject();
}
else {
def.resolve(data); //pass the data to the .then() handler.
}
});
return def.promise;
}
function someAsyncFuncName2() {
var def = q.defer();
//async function
setTimeout(function(){ //suppose
//do something
if(good) {
def.resolve();
} else {
def.reject();
}
}, 1000); //arbitrary timeout of 1 second
return def.promise;
}
USAGE:
q.all([someAsyncFuncName1('/api-1'), someAsyncFuncName2()])
.then(function() {
//final handler
});
On a similar line of thought one can use q.allSettled() if you want to wait for all promises to return.
Hope this helps.
---EOF OLD ANSWER---

First of all, if async functions used in PromiseA don't return promises, you need to promisify them. You can do that with Promise constructor, but it's much better to use libraries, such as bluebird with their promisify methods.
Let's imagine, that we have two functions getUserIdsAsync and getUserAsync. The first on returns a list of user ids, getUserAsync returns an user data by userId. And you need to get a list of users by their ids. The code of PromiseA could look so:
var promiseA = function() {
return getUserIdsAsync()
.then(userIds => {
let ops = users.map(uid => getUserAsync(uid));
return Promise.all(ops);
});
}

The following snippet shows a solution without using any external library like bluebird. It follows the code snippet in your question (which seems to be more complicate than needed).
You have to collect all api promisses in an array. Then you can call Promise.all() to get a Promise for the end of all api promisses. Then you can do some final stuff, like parsing the result of each promise and continue afterwards.
function getRandomInt(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
var apiEndpoint = function (name) {
return new Promise( (resolve, reject) => {
setTimeout(() => resolve('API ' + name + ' job done'), 1000);
});
}
var promiseA = function() {
return new Promise( (resolve, reject) => {
const promisses = [];
for (var i=1; i < getRandomInt(3,6); i++) {
// 1. Establish objects needed from one API endpoint
promisses.push(apiEndpoint('This is number ' + i));
}
Promise.all(promisses).then( results => {
// do final stuff
for (const s of results) {
// 2. Call API endpoint for each object and parse
console.log(s);
}
// continue ...
// 3. Only then continue to next promise
resolve('now it is finished');
}).catch( err => reject(err) );
});
}
var finalPromise = function() {
return new Promise( (resolve, reject) => {
console.log('finalPromise');
resolve();
});
}
promiseA()
.then( () => finalPromise())
.catch(err => console.log(err) );
Please hold in mind that this solution is not easy to read. Using external libraries or reducing promisses can improve readability. Maybe you should take a look to the async/await pattern to get a much more better (readable) solution.
Here is a solution with async/await:
function getRandomInt(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
const apiEndpoint = function (name) {
return new Promise( (resolve, reject) => {
setTimeout(() => resolve('API ' + name + ' job done'), 1000);
});
}
async function promiseParallel () {
const promisses = [];
for (let i = 1; i < getRandomInt(3,6); i++) {
promisses.push(apiEndpoint('This is number ' + i));
}
for (const p of promisses) {
const x = await p;
console.log(x);
}
return ('everything is done');
}
promiseParallel().then( result => {
console.log(result);
}).catch( err => console.log(err) );
If you want call the promisses sequentially you can replace with:
async function promiseSequ () {
for (let i = 1; i < getRandomInt(3,6); i++) {
const x = await apiEndpoint('This is number ' + i);
console.log(x);
}
return ('everything is done');
}

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 */);

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>