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How can I execute some async tasks in parallel with limit in generator function?

I'm trying to execute some async tasks in parallel with a limitation on the maximum number of simultaneously running tasks.
There's an example of what I want to achieve:
Currently this tasks are running one after another. It's implemented this way:
export function signData(dataItem) {
cadesplugin.async_spawn(async function* (args) {
//... nestedArgs assignment logic ...
for (const id of dataItem.identifiers) {
yield* idHandler(dataItem, id, args, nestedArgs);
}
// some extra logic after all tasks were finished
}, firstArg, secondArg);
}
async function* idHandler(edsItem, researchId, args, nestedArgs) {
...
let oDocumentNameAttr = yield cadesplugin.CreateObjectAsync("CADESCOM.CPAttribute");
yield oDocumentNameAttr.propset_Value("Document Name");
...
// this function mutates some external data, making API calls and returns void
}
Unfortunately, I can't make any changes in cadesplugin.* functions, but I can use any external libraries (or built-in Promise) in my code.
I found some methods (eachLimit and parallelLimit) in async library that might work for me and an answer that shows how to deal with it.
But there are still two problems I can't solve:
How can I pass main params into nested function?
Main function is a generator function, so I still need to work with yield expressions in main and nested functions
There's a link to cadesplugin.* source code, where you can find async_spawn (and another cadesplugin.*) function that used in my code.
That's the code I tried with no luck:
await forEachLimit(dataItem.identifiers, 5, yield* async function* (researchId, callback) {
//... nested function code
});
It leads to Object is not async iterable error.
Another attempt:
let functionArray = [];
dataItem.identifiers.forEach(researchId => {
functionArray.push(researchIdHandler(dataItem, id, args, nestedArgs))
});
await parallelLimit(functionArray, 5);
It just does nothing.
Сan I somehow solve this problem, or the generator functions won't allow me to do this?

square peg, round hole
You cannot use async iterables for this problem. It is the nature of for await .. of to run in series. await blocks and the loop will not continue until the awaited promise has resovled. You need a more precise level of control where you can enforce these specific requirements.
To start, we have a mock myJob that simulates a long computation. More than likely this will be a network request to some API in your app -
// any asynchronous task
const myJob = x =>
sleep(rand(5000)).then(_ => x * 10)
Using Pool defined in this Q&A, we instantiate Pool(size=4) where size is the number of concurrent threads to run -
const pool = new Pool(4)
For ergonomics, I added a run method to the Pool class, making it easier to wrap and run jobs -
class Pool {
constructor (size) ...
open () ...
deferNow () ...
deferStacked () ...
// added method
async run (t) {
const close = await this.open()
return t().then(close)
}
}
Now we need to write an effect that uses our pool to run myJob. Here you will also decide what to do with the result. Note the promise must be wrapped in a thunk otherwise pool cannot control when it begins -
async function myEffect(x) {
// run the job with the pool
const r = await pool.run(_ => myJob(x))
// do something with the result
const s = document.createTextNode(`${r}\n`)
document.body.appendChild(s)
// return a value, if you want
return r
}
Now run everything by mapping myEffect over your list of inputs. In our example myEffect we return r which means the result is also available after all results are fetched. This optional but demonstrates how program knows when everything is done -
Promise.all([1,2,3,4,5,6,7,8,9,10,11,12].map(myEffect))
.then(JSON.stringify)
.then(console.log, console.error)
full program demo
In the functioning demo below, I condensed the definitions so we can see them all at once. Run the program to verify the result in your own browser -
class Pool {
constructor (size = 4) { Object.assign(this, { pool: new Set, stack: [], size }) }
open () { return this.pool.size < this.size ? this.deferNow() : this.deferStacked() }
async run (t) { const close = await this.open(); return t().then(close) }
deferNow () { const [t, close] = thread(); const p = t.then(_ => this.pool.delete(p)).then(_ => this.stack.length && this.stack.pop().close()); this.pool.add(p); return close }
deferStacked () { const [t, close] = thread(); this.stack.push({ close }); return t.then(_ => this.deferNow()) }
}
const rand = x => Math.random() * x
const effect = f => x => (f(x), x)
const thread = close => [new Promise(r => { close = effect(r) }), close]
const sleep = ms => new Promise(r => setTimeout(r, ms))
const myJob = x =>
sleep(rand(5000)).then(_ => x * 10)
async function myEffect(x) {
const r = await pool.run(_ => myJob(x))
const s = document.createTextNode(`${r}\n`)
document.body.appendChild(s)
return r
}
const pool = new Pool(4)
Promise.all([1,2,3,4,5,6,7,8,9,10,11,12].map(myEffect))
.then(JSON.stringify)
.then(console.log, console.error)
slow it down
Pool above runs concurrent jobs as quickly as possible. You may also be interested in throttle which is also introduced in the original post. Instead of making Pool more complex, we can wrap our jobs using throttle to give the caller control over the minimum time a job should take -
const throttle = (p, ms) =>
Promise.all([ p, sleep(ms) ]).then(([ value, _ ]) => value)
We can add a throttle in myEffect. Now if myJob runs very quickly, at least 5 seconds will pass before the next job is run -
async function myEffect(x) {
const r = await pool.run(_ => throttle(myJob(x), 5000))
const s = document.createTextNode(`${r}\n`)
document.body.appendChild(s)
return r
}

In general, it should be better to apply #Mulan answer.
But if you also stuck into cadesplugin.* generator functions and don't really care about heavyweight external libraries, this answer may also be helpful.
(If you are worried about heavyweight external libraries, you may still mix this answer with #Mulan's one)
Async task running can simply be solved using Promise.map function from bluebird library and double-usage of cadesplugin.async_spawn function.
The code will look like the following:
export function signData(dataItem) {
cadesplugin.async_spawn(async function* (args) {
// some extra logic before all of the tasks
await Promise.map(dataItem.identifiers,
(id) => cadesplugin.async_spawn(async function* (args) {
// ...
let oDocumentNameAttr = yield cadesplugin.CreateObjectAsync("CADESCOM.CPAttribute");
yield oDocumentNameAttr.propset_Value("Document Name");
// ...
// this function mutates some external data and making API calls
}),
{
concurrency: 5 //Parallel tasks count
});
// some extra logic after all tasks were finished
}, firstArg, secondArg);
}
The magic comes from async_spawn function which is defined as:
function async_spawn(generatorFunction) {
async function continuer(verb, arg) {
let result;
try {
result = await generator[verb](arg);
} catch (err) {
return Promise.reject(err);
}
if (result.done) {
return result.value;
} else {
return Promise.resolve(result.value).then(onFulfilled, onRejected);
}
}
let generator = generatorFunction(Array.prototype.slice.call(arguments, 1));
let onFulfilled = continuer.bind(continuer, "next");
let onRejected = continuer.bind(continuer, "throw");
return onFulfilled();
}
It can suspend the execution of internal generator functions on yield expressions without suspending the whole generator function.

Using promise in loop results in Promise failure

I'd like to reuse the same code in a loop. This code contains promises. However, when iterating, this code results in an error.
I've tried using for and while loops. There seems to be no issue when I use the for loop for a single iteration.
Here is a minimal version of my code:
var search_url = /* Some initial URL */
var glued = "";
for(var i = 0; i < 2; i++)
{
const prom = request(search_url)
.then(function success(response /* An array from a XMLHTTPRequest*/) {
if (/* Some condition */)
{
search_url = /* Gets next URL */
glued += processQuery(response[0]);
} else {
console.log("Done.")
}
})
.catch(function failure(err) {
console.error(err.message); // TODO: do something w error
})
}
document.getElementById('api-content').textContent = glued;
I expect the results to append to the variable glued but instead, I get an error: failure Promise.catch (async) (anonymous) after the first iteration of the loop.

Answer:
You can use the Symbol.iterator in accordance with for await to perform asynchronous execution of your promises. This can be packaged up into a constructor, in the example case it's called Serial (because we're going through promises one by one, in order)
function Serial(promises = []) {
return {
promises,
resolved: [],
addPromise: function(fn) {
promises.push(fn);
},
resolve: async function(cb = i => i, err = (e) => console.log("trace: Serial.resolve " + e)) {
try {
for await (let p of this[Symbol.iterator]()) {}
return this.resolved.map(cb);
} catch (e) {
err(e);
}
},
[Symbol.iterator]: async function*() {
this.resolved = [];
for (let promise of this.promises) {
let p = await promise().catch(e => console.log("trace: Serial[Symbol.iterator] ::" + e));
this.resolved.push(p);
yield p;
}
}
}
}
What is the above?
It's a constructor called Serial.
It takes as an argument an array of Functions that return Promises.
The functions are stored in Serial.promises
It has an empty array stored in Serial.resolved - this will store the resolved promise requests.
It has two methods:
addPromise: Takes a Function that returns a Promise and adds it to Serial.promises
resolve: Asynchronously calls a custom Symbol.iterator. This iterator goes through every single promise, waits for it to be completed, and adds it to Serial.resolved. Once this is completed, it returns a map function that acts on the populated Serial.resolved array. This allows you to simply call resolve and then provide a callback of what to do with the array of responses. A.e. .resolve()((resolved_requests) => //do something with resolved_requests)
Why does it work?
Although many people don't realize this Symbol.iterator is much more powerful than standard for loops. This is for two big reasons.
The first reason, and the one that is applicable in this situation, is because it allows for asynchronous calls that can affect the state of the applied object.
The second reason is that it can be used to provide two different types of data from the same object. A.e. You may have an array that you would like to read the contents of:
let arr = [1,2,3,4];
You can use a for loop or forEach to get the data:
arr.forEach(v => console.log(v));
// 1, 2, 3, 4
But if you adjust the iterator:
arr[Symbol.iterator] = function* () {
yield* this.map(v => v+1);
};
You get this:
arr.forEach(v => console.log(v));
// 1, 2, 3, 4
for(let v of arr) console.log(v);
// 2, 3, 4, 5
This is useful for many different reasons, including timestamping requests/mapping references, etc. If you'd like to know more please take a look at the ECMAScript Documentation: For in and For Of Statements
Use:
It can be used by calling the constructor with an Array of functions that return Promises. You can also add Function Promises to the Object by using
new Serial([])
.addPromise(() => fetch(url))
It doesn't run the Function Promises until you use the .resolve method.
This means that you can add promises ad hoc if you'd like before you do anything with the asynchronous calls. A.e. These two are the same:
With addPromise:
let promises = new Serial([() => fetch(url), () => fetch(url2), () => fetch(url3)]);
promises.addPromise(() => fetch(url4));
promises.resolve().then((responses) => responses)
Without addPromise:
let promises = new Serial([() => fetch(url), () => fetch(url2), () => fetch(url3), () => fetch(url4)])
.resolve().then((responses) => responses)
Data:
Since I can't really replicate your data calls, I opted for JSONPlaceholder (a fake online rest api) to show the promise requests in action.
The data looks like this:
let searchURLs = ["https://jsonplaceholder.typicode.com/todos/1",
"https://jsonplaceholder.typicode.com/todos/2",
"https://jsonplaceholder.typicode.com/todos/3"]
//since our constructor takes functions that return promises, I map over the URLS:
.map(url => () => fetch(url));
To get the responses we can call the above data using our constructor:
let promises = new Serial(searchURLS)
.resolve()
.then((resolved_array) => console.log(resolved_array));
Our resolved_array gives us an array of XHR Response Objects. You can see that here:
function Serial(promises = []) {
return {
promises,
resolved: [],
addPromise: function(fn) {
promises.push(fn);
},
resolve: async function(cb = i => i, err = (e) => console.log("trace: Serial.resolve " + e)) {
try {
for await (let p of this[Symbol.iterator]()) {}
return this.resolved.map(cb);
} catch (e) {
err(e);
}
},
[Symbol.iterator]: async function*() {
this.resolved = [];
for (let promise of this.promises) {
let p = await promise().catch(e => console.log("trace: Serial[Symbol.iterator] ::" + e));
this.resolved.push(p);
yield p;
}
}
}
}
let searchURLs = ["https://jsonplaceholder.typicode.com/todos/1", "https://jsonplaceholder.typicode.com/todos/2", "https://jsonplaceholder.typicode.com/todos/3"].map(url => () => fetch(url));
let promises = new Serial(searchURLs).resolve().then((resolved_array) => console.log(resolved_array));
Getting Results to Screen:
I opted to use a closure function to simply add text to an output HTMLElement.
This is added like this:
HTML:
<output></output>
JS:
let output = ((selector) => (text) => document.querySelector(selector).textContent += text)("output");
Putting it together:
If we use the output snippet along with our Serial object the final functional code looks like this:
let promises = new Serial(searchURLs).resolve()
.then((resolved) => resolved.map(response =>
response.json()
.then(obj => output(obj.title))));
What's happening above is this:
we input all our functions that return promises. new Serial(searchURLS)
we tell it to resolve all the requests .resolve()
after it resolves all the requests, we tell it to take the requests and map the array .then(resolved => resolved.map
the responses we turn to objects by using .json method. This is necessary for JSON, but may not be necessary for you
after this is done, we use .then(obj => to tell it to do something with each computed response
we output the title to the screen using output(obj.title)
Result:
let output = ((selector) => (text) => document.querySelector(selector).textContent += text)("output");
function Serial(promises = []) {
return {
promises,
resolved: [],
addPromise: function(fn) {
promises.push(fn);
},
resolve: async function(cb = i => i, err = (e) => console.log("trace: Serial.resolve " + e)) {
try {
for await (let p of this[Symbol.iterator]()) {}
return this.resolved.map(cb);
} catch (e) {
err(e);
}
},
[Symbol.iterator]: async function*() {
this.resolved = [];
for (let promise of this.promises) {
let p = await promise().catch(e => console.log("trace: Serial[Symbol.iterator] ::" + e));
this.resolved.push(p);
yield p;
}
}
}
}
let searchURLs = ["https://jsonplaceholder.typicode.com/todos/1", "https://jsonplaceholder.typicode.com/todos/2", "https://jsonplaceholder.typicode.com/todos/3"].map(url => () => fetch(url));
let promises = new Serial(searchURLs).resolve()
.then((resolved) => resolved.map(response =>
response.json()
.then(obj => output(obj.title))));
<output></output>
Why go this route?
It's reusable, functional, and if you import the Serial Constructor you can keep your code slim and comprehensible. If this is a cornerstone of your code, it'll be easy to maintain and use.
Using it with your code:
I will add how to specifically use this with your code to fully answer your question and so that you may understand further.
NOTE glued will be populated with the requested data, but it's unnecessary. I left it in because you may have wanted it stored for a reason outside the scope of your question and I don't want to make assumptions.
//setup urls:
var search_urls = ["https://jsonplaceholder.typicode.com/todos/1", "https://jsonplaceholder.typicode.com/todos/2"];
var request = (url) => () => fetch(url);
let my_requests = new Serial(search_urls.map(request));
//setup glued (you don't really need to, but if for some reason you want the info stored...
var glued = "";
//setup helper function to grab title(this is necessary for my specific data)
var addTitle = (req) => req.json().then(obj => (glued += obj.title, document.getElementById('api-content').textContent = glued));
// put it all together:
my_requests.resolve().then(requests => requests.map(addTitle));
Using it with your code - Working Example:
function Serial(promises = []) {
return {
promises,
resolved: [],
addPromise: function(fn) {
promises.push(fn);
},
resolve: async function(cb = i => i, err = (e) => console.log("trace: Serial.resolve " + e)) {
try {
for await (let p of this[Symbol.iterator]()) {}
return this.resolved.map(cb);
} catch (e) {
err(e);
}
},
[Symbol.iterator]: async function*() {
this.resolved = [];
for (let promise of this.promises) {
let p = await promise().catch(e => console.log("trace: Serial[Symbol.iterator] ::" + e));
this.resolved.push(p);
yield p;
}
}
}
}
//setup urls:
var search_urls = ["https://jsonplaceholder.typicode.com/todos/1", "https://jsonplaceholder.typicode.com/todos/2"];
var request = (url) => () => fetch(url);
let my_requests = new Serial(search_urls.map(request));
//setup glued (you don't really need to, but if for some reason you want the info stored...
var glued = "";
//setup helper function to grab title(this is necessary for my specific data)
var addTitle = (req) => req.json().then(obj => (glued += obj.title, document.getElementById('api-content').textContent = glued));
// put it all together:
my_requests.resolve().then(requests => requests.map(addTitle));
<div id="api-content"></div>
Final Note
It's likely that we will be seeing a prototypal change to the Promise object in the future that allows for easy serialization of Promises. Currently (7/15/19) there is a TC39 Proposal that does add a lot of functionality to the Promise object but it hasn't been fully vetted yet, and as with many ideas trapped within the Proposal stage, it's almost impossible to tell when they will be implemented into Browsers, or even if the idea will stagnate and fall off the radar.
Until then workarounds like this are necessary and useful( the reason why I even went through the motions of constructing this Serializer object was for a transpiler I wrote in Node, but it's been very helpful beyond that! ) but do keep an eye out for any changes because you never know!
Hope this helps! Happy Coding!

Your best bet is probably going to be building up that glued variable with recursion.
Here's an example using recursion with a callback function:
var glued = "";
requestRecursively(/* Some initial URL string */, function() {
document.getElementById('api-content').textContent = glued;
});
function requestRecursively(url, cb) {
request(url).then(function (response) {
if (/* Some condition */) {
glued += processQuery(response[0]);
var next = /* Gets next URL string */;
if (next) {
// There's another URL. Make another request.
requestRecursively(next, cb);
} else {
// We're done. Invoke the callback;
cb();
}
} else {
console.log("Done.");
}
}).catch(function (err) {
console.error(err.message);
});
}

Loops and Callback hell

Suppose you have an Array/Object that contains a list of values. Lets say those a mysql commands or urls or filespaths. Now you want to iterate over all of them and execute some code over every entry.
for(let i = 0; i < urls.length; i++){
doSthWith(urls[i]);
}
No Problem so far. But now lets say each function has a callback and needs the result of the last execution. e.g. you request something from one website and you want to use the results of this request for one of your following requests.
for(let i = 0; i < urls.length; i++){
if(resultOfLastIteration.successful){ //or some other result besides the last one
doSthWith(urls[i]);
}
}
Now lets say the length of urls (or sth similar) is over 100. Thats why you normaly use a loop so you dont need to write the same function a 100 times. That also means that Promises wont do the trick either (except Im unaware trick a trick), because you have the same problem:
doSthWith(urls[0]).then(...
doSthWith(urls[1]).then(... //either put them inside each other
).then(...
doSthWith(urls[i]) //or in sequence
...
).catch(err){...}
Either way I dont see a way to use a loop.
A way that I found but isnt really "good" is to use the package "wait.for"(https://www.npmjs.com/package/wait.for). But what makes this package tricky is to launch a fiber each time you want to use wait.for:
//somewhere you use the function in a fiber Context
wait.for(loopedExecutionOfUrls, urls);
//function declaration
function loopedExecutionOfUrls(urls, cb){
//variables:
for(let i = 0; i < urls.length; i++){
if(someTempResultVar[i-1] === true){
someTempResultVar = wait.for(doSthWith,urls[i]);
} else if(...){...}
}
}
But Im not sure if this approach is really good, besides you always have to check if you have wrapped the whole thing in a Fiber so for each function that has loops with functions that have callbacks. Thus you have 3 levels: the lauchFiber level, wait.for(loopedFunction) level and the wait.for the callback function level. (Hope I that was formulated understandable)
So my questions is: Do you guys have a good approach where you can loop throw callback functions and can use results of those whenever you like?
good = easy to use, read, performant, not recursive,...
(Im sorry if this question is stupid, but I really have problems getting along with this asynchronous programming)

If you want to wait for doSthWith to finish before doing the same but with the nex url, you have to chain your promises and you can use array.prototype.reduce to do that:
urls = ["aaa", "bbb", "ccc", "ddd"];
urls.reduce((lastPromise, url) => lastPromise.then((resultOfPreviousPromise) => {
console.log("Result of previous request: ", resultOfPreviousPromise); // <-- Result of the previous request that you can use for the next request
return doSthWith(url);
}), Promise.resolve());
function doSthWith(arg) { // Simulate the doSthWith promise
console.log("do something with: ", arg);
return new Promise(resolve => {
setTimeout(() => resolve("result of " + arg), 2000);
});
}

Use async, specifically async.each:
const async = require('async');
function doSthWith(url, cb) {
console.log('doing something with ' + url);
setTimeout(() => cb(), 2000);
}
const urls = ['https://stackoverflow.com/', 'https://phihag.de/'];
async.each(urls, doSthWith, (err) => {
if (err) {
// In practice, likely a callback or throw here
console.error(err);
} else {
console.log('done!');
}
});
Use async.map if you are interested in the result.

When I need to loop over promises I use my handy dandy ploop function. Here is an example:
// Function that returns a promise
var searchForNumber = function(number) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
var min = 1;
var max = 10;
var val = Math.floor(Math.random()*(max-min+1)+min);
console.log('Value is: ' + val.toString());
return resolve(val);
}, 1000);
});
};
// fn : function that should return a promise.
// args : the arguments that should be passed to fn.
// donefn : function that should check the result of the promise
// and return true to indicate whether ploop should stop or not.
var ploop = function(fn, args, donefn) {
return Promise.resolve(true)
.then(function() {
return(fn.apply(null, args));
})
.then(function(result) {
var finished = donefn(result);
if(finished === true){
return result;
} else {
return ploop(fn, args, donefn);
}
});
};
var searchFor = 4;
var donefn = function(result) {
return result === searchFor;
};
console.log('Searching for: ' + searchFor);
ploop(searchForNumber, [searchFor], donefn)
.then(function(val) {
console.log('Finally found! ' + val.toString());
process.exit(0);
})
.catch(function(err) {
process.exit(1);
});

sequential call using promise inside loop

i want to perform synchronous operation of functions using promise. I have loop that passes the data to be inserted to insert function and after inserting one row i want to check the no. of rows exists in table hence i am performing select operation.
But the issue is if there are 3 records then it inserts all 3 records and after that my select function gets executed. what i want is after insertion of one record select function gets called.
here is my pseudo code as entire code involves lot of operations
for(var i=0; data.length ; i++){
self.executeFeedbackTrack(data);
}
executeFeedbackTrack:function(callInfo){
var self=this;
return self.insertFeedbackTrack(callInfo).then(function(data){
console.log("insertFeedbackTrack status "+status);
return self.getFeedbackTrack();
});
},
getFeedbackTrack :function(){
return new Promise(function(resolve,reject){
var objDBFeedbackTrack = new DBFeedbackTrack();
objDBFeedbackTrack.selectFeedbackTrack(function(arrayCallRegisters){
if(arrayCallRegisters){
console.log("notification.js no. of feedbacks "+arrayCallRegisters.length);
resolve(arrayCallRegisters.length);
}
});
});
},
insertFeedbackTrack :function(callInfo){
return new Promise(function(resolve,reject){
var objDBFeedbackTrack = new DBFeedbackTrack();
objDBFeedbackTrack.insertFeedbackTrack(callInfo.callNumber,callInfo.callServiceType,function(status){
resolve(status);
$('#loader').hide();
});
});
}

The previous answer is good, but if you are using nodejs, or babel, or you are using only modern browsers. You can use an async-await pair, it is es8 stuff.
let insertFeedbackTrack = function(){ return new Promise(/***/)};
let getFeedbackTrack = function(){ return new Promise(/***/)};
let processResult = async function(data){
let feedbacks = [];
for(let i=0;i<data.length;i++){
let insertedResult = await insertFeedbackTrack(data[i]);//perhaps you will return an id;
let feedbackTrack = await getFeedbackTrack(insertedResult.id);
feedbacks.push(feedbackTrack);
}
return feedbacks;
}
processResult(data).then(/** do stuff */)

It looks to me like this is caused by executing a series of asynchronous inserts, and assuming that the get of insert n (inside of a .then()) is called before insert n+1 is executed. However, I'm not aware of any such guarantee, in JavaScript; all that I'm familiar with is that then n will be called after insert n, not that it would be called before insert n+1.
What I'd suggest is avoiding this mix of traditional and callback-based code, and instead put the iteration step inside getFeedbackTrack().then. Assuming this understanding of the issue is correct, then something like the following should work:
function iterate(i) {
if (i < data.length) {
obj.insertFeedbackTrack(data[i]).then(function(insertResult) {
self.getFeedbackTrack().then(function(getResult) {
// this line is the important one, replacing the `for` loop earlier
iterate(i+1);
});
});
}
}
iterate(0);
By doing that, you would guarantee that insert for the next element does not occur until the current select executes successfully.
Naturally, you may also want to restructure that to use chained .then instead of nested; I used nested rather than chained to emphasize the ordering of callbacks.

This can be solved by using a very handy JS library Ramda. Concept is to use two methods, one is R.partial and another is R.pipeP.
First create a promises array from your data array, like following.
var promises = data.map(function(i) {
return R.partial(sample, [i])
});
Then you can pass this promise to R.pipeP, so that it can be executed one after another. like below.
var doOperation = R.pipeP.apply(this, promises)
Please execute following snippet attached.
// Sample promise returning function
function sample(d) {
return new Promise(function(resolve, reject){
setTimeout(function() {
console.log('resolved for:' + d);
resolve(d);
}, 1000)
})
}
// Sample data
var data = [1, 2, 3, 4, 5]
// Converting data array to promise array
var promises = data.map(function(i) {
return R.partial(sample, [i])
});
var doOperation = R.pipeP.apply(this, promises)
doOperation();
<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.25.0/ramda.min.js"></script>
So in your case, the code will look like this
var promises = data.map(function(i) {
return R.partial(self.executeFeedbackTrack, [i])
});
var doOperation = R.pipeP.apply(this, promises)
doOperation();

I use yield for such cases if using generator functions.
for(var i = 0; i < order.tasks.length; i++){
if(order.tasks[i].customer_id === 0){
var name = order.tasks[i].customer_name.split(" ")
const customers = yield db.queryAsync(
`INSERT INTO customers(
business_id)
VALUES(?)
`,[order.business_id])
}
}
Or else I use self-calling functions in case of callbacks.
var i = 0;
(function loop() {
if (i < data.length) {
task_ids1.push([order.tasks[i].task_id])
i++;
loop();
}
}());

Here's how I would sequentially call promises in a loop (I'm using ES7).
First, let's define some basic data:
const data = [0,1,2,3];
Then, let's simulate some long running process, so let's create a function that returns a Promise (you can think of this as a simulated network request, or whatever suits your needs)
const promiseExample = (item) =>
new Promise((res) => {
setTimeout(() => {
console.log('resolved ', item);
res(item);
}, 1000);
});
Now, let's create an array of promises. What the next line of code does is: for every item in the array data, return a promise factory. A promise factory is a function that wraps a certain promise without running it.
const funcs = data.map(item => async () => await promiseExample(item));
Now, the actual code starts here. We need a function that does the actual serialization. Since it has to handle an array of promiseFactories, I split it in two functions, one for the serialization of a single promise, and one for handling an array of promiseFactories.
const serializePromise = promiseFactoryList =>
promiseFactoryList.reduce(serialize, Promise.resolve([]));
const serialize = async (promise, promiseFactory) => {
const promiseResult = await promise;
const res = await promiseFactory();
return [...promiseResult, res];
};
Now, you can simply call it like this:
serializePromise(funcs).then(res => {
console.log('res', res);
});
As you can see, the code is pretty simple, elegant, functional, and doesn't need any external dependency. I hope this answers your question and helps you!
const serializePromise = promiseFactoryList =>
promiseFactoryList.reduce(serialize, Promise.resolve([]));
const serialize = async (promise, promiseFactory) => {
const promiseResult = await promise;
const res = await promiseFactory();
return [...promiseResult, res];
};
const data = [0,1,2,3];
const promiseExample = (item) =>
new Promise((res) => {
setTimeout(() => {
console.log('resolved ', item);
res(item);
}, 1000);
});
const funcs = data.map(item => async () => await promiseExample(item))
serializePromise(funcs).then(res => {
console.log('res', res);
});

I ran into this problem recently and solved it as shown below. This is very similar to the answer by #Ethan Kaminsky, but only uses callbacks. This may be useful for people avoiding promises for whatever reason.
In my application the asynchronous function may fail and can safely be retried; I included this logic because it's useful and doesn't overly complicate the routine, but it is not exercised in the example.
// Some callback when the task is complete
function cb(...rest) { window.alert( `${rest.join(', ')}` ) }
// The data and the function operating on the data
// The function calls "callback(err)" on completion
const data = [ 'dataset1', 'dataset2', 'dataset3' ]
const doTheThing = (thingDone) => setTimeout( thingDone, 1000 )
let i = -1 // counter/interator for data[]
let retries = 20 // everything fails; total retry #
// The do-async-synchronously (with max retries) loop
function next( err ) {
if( err ) {
if( ! --retries ) return cb( 'too many retries' )
} else if( ! data[++i] ) return cb( undefined, 'done-data' )
console.log( 'i is', i, data[i] )
doTheThing( next, data[i] ) // callback is first here
}
// start the process
next()

How to return a result from a generator calling an asynchronous function?

I'm using the Koa framework and ES6. How do I get the first method to return a result to this view function that requires a yield? I am trying to use the pg library, which uses asynchronous calls, in a node 7.5 app.
pgrepo.listCities = function*() {
pool.query('SELECT distinct(town) from public.property_uk', function(err, result) {
console.log("result: " + JSON.stringify(result.rows));
// What now?
});
};
www.pgindex = function*() {
let results = yield pgrepo.listCities(); // What now?
console.log('handler: ' + results)
yield this.render('pgindex', {
items: results
});
}
I understand what is happening, the yield is deferring execution, so the function runs and yields with no results, and then the query fires and produces results.
So, my question is, how can I refactor these 2 functions so that the query results are returned to the function that wants to pass them to the view.

Wasn't thinking of answering because I have no idea about Koa / generators and what not, but can't you pull this off with simple promise? (Since no body else is posting, figured at least to try to give you some way that you might be able to solved it)
This code is not tested, and I have no idea about your stack, so this is only here as possible solution in case that you have a bit more freedom with code
// Since I have no idea why generators should be used here, let's remove the *
// if you really do need it, then this answer might not work (as I said, I never
// even touched generator functions, so I will just make this as arrow fn)
pgrepo.listCities = () => {
// first let's return a promise to function call
return new Promise((resolve, reject) => {
pool.query('SELECT distinct(town) from public.property_uk', function(err, result) {
// since this is in callback, we can be sure that this will be executed
// when the query itself is finished. because this is a promise we
// can now here resolve it here with results. but first check for
// errors and reject if this query failed
if (err) {
reject(err);
}
resolve(result.rows);
});
});
};
www.pgindex = () => {
// Since I have no idea why generators should be used here, let's remove the
// yield...
let results = pgrepo.listCities;
// now our results can call the listCities and get the promise
// back as result. as soon as that promise is resolved we can enter into .then
// and access values
results()
.then((results) => {
// this console log should now have all the results from the query.
console.log(results);
// you can now do here what you want with the data. again not sure how
// this can play with generators
// also this (both this as next commented block and this as pointer) will
// most likely fail due to arrow function changing context of this, so you
// might wanna play around with that to be sure you have correct context
// if it doesn't work out of the box. If you can't figure it you, you can
// change this arrow function in normal 'function (results) {...}' and your
// context will be fine and you will be able to call proper this (I think :) )
this.render('pgindex', {
items: results
});
}) // and ofc error handling
.catch((err) => {
console.log('Something went wrong and I am terrible error message ' + err);
});
};

What you're probably looking for is the co-routines version of your code which make your promises look like syncronous code by hiding its async nature. Instead .thening promises, you can yield them, and have your code would look like this:
let co = require('co');
let pg = require('co-pg')(require('pg'));
let connectionString = 'postgres://postgres:1234#localhost/postgres';
co(function* connectExample() {
try {
let client = new pg.Client(connectionString);
yield client.connectPromise();
let result = yield client.queryPromise('SELECT distinct(town) from public.property_uk');
console.log(result);
client.end();
} catch (ex) {
console.error(ex);
}
});
This is similar to async/await but you can use it today! (although at the time of writing, async/await is entering Node very soon)
Notice that I've promisified the pg module so it can be used with co

Without the need to reinvent anything, pg-promise supports ES6 generators out of the box:
var pgp = require('pg-promise')(/*initialization options*/);
var db = pgp(/*your connection details*/);
function * getCities(t) {
var cities = yield t.query('SELECT distinct(town) from public.property_uk');
// do whatever you need here
return cities;
}
function * Generator_Caller() {
try {
var cities = yield db.task(getCities);
console.log(cities);
}
catch (error) {
console.log(error);
}
}
function Regular_Caller() {
db.task(getCities)
.then(function (cities) {
console.log(cities);
})
.catch(function (error) {
console.log(error);
});
}

thanks for all the help, the final code I went with that worked is:
pgrepo.listTowns = function*() {
var results = yield pool.query('SELECT distinct(town) from public.property_uk');
return results; };
www.pgindex = function*() {
const results = yield pgrepo.listTowns();
console.log("results: " + JSON.stringify(results.rows));
yield this.render('pgindex', {
items: results.rows
});};