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How do I make a nested loop continue only after a asynchronous function has been resolved or how do I extend ".then" beyond the scope

I tried to prevent async problems with promises in the following code. By using a .then function everything within that function gets called after the function has been resolved. But now I have the problem that neither can I extend the scope of the ".then function" enough to include the bits after the second loop nor can I to my knowledge easily pause the code until the function has been properly resolved and THEN continue with the loop iteration.
Here's my main code(simplified):
let total = []
$.each(element, function(data) {
//Some other code
let out;
$.each(element2, function(data2) {
getZip(data2).then(function(txt){ //after everything has finished this get's called
out = someFunction(txt,data2);
total.push(out);
});
)};
console.log(total)//this gets called first
//some other code that does some stuff with total
)};
Here's the getZip code which is asynchronous:
function getZip(zipFile) {
return new Promise(function (resolve, reject){
zip = new JSZip()
JSZipUtils.getBinaryContent("someURL/" + zipFile, function (err, data) {
if (err) {
reject(err)
}
JSZip.loadAsync(data).then(function (zip) {
return zip.file(zipFile.replace(".zip", "")).async("text"); //gets the file within the zip andoutputs as text
}).then(function (txt) {
resolve(txt)
});
});
});
}
I'd be happy if either the getZip code could be made synchronous or if the before mentioned could be done.

I do not think I fully understand the code you have written. However, I recommend you use Promise.all. Here is an example I have written that I hope helps guide you:
let total = [];
$.each([1,2,3,4], function (data) {
// Some other code.
let out;
// Create a new promise so that we can wait on the getZip method.
new Promise(function (resolve, reject) {
// Create a holder variable. This variable with hold all the promises that are output from the getZip method you have.
let gZipPromises = [];
$.each([5,6,7,8], function (data2) {
// Your getZip method would go here. wrap the call to getZip in gZipPromises.push to push all the returned promises onto the holding variable.
gZipPromises.push(new Promise(function (resolve2, reject2) {
// Sample Code
setTimeout(function () {
total.push(data2);
resolve2("");
}, 10);
// End Sample Code.
}));
});
// Pass the holding variable to Promise.all so that all promises in the holding variable are executed before resolving.
Promise.all(gZipPromises).then(function() {
resolve()
});
}).then(function () {
// This will be called only when all getZip promises are completed in the second loop.
console.log(total);
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
With that said, I could not test your code. But I think this would work:
(Please note that based on the code you provided, the variable total would be logged for each iteration of the top most $.each
let total = []
$.each(element, function(data) {
//Some other code
let out;
// Define a new promise.
new Promise(function (resolve, reject) {
let gZipPromises = [];
$.each(element2, function(data2) {
gZipPromises.push(
getZip(data2).then(function(txt){ //after everything has finished this get's called
out = someFunction(txt,data2);
total.push(out);
});
);
)};
Promise.all(gZipPromises).then(function() {
resolve()
});
}).then(function () {
console.log(total)
});
)};

const elements = [["foo.zip"],["bar.zip"],["baz.zip"]];
const totalOut = getAllZips(elements)
.then(text => console.info(text))
.catch(error => console.error(error))
function someFunction(text, data) {
return `${text}\nLength: ${data.length}`;
}
async function getAllZips(elements) {
let promises = [];
for(const element of elements) {
for(const data of element) {
promises.push(getZip(data).then(text => {
return someFunction(text, data);
}));
}
}
return Promise.all(promises);
}
async function getZip(file) {
return new Promise((resolve, reject) => {
JSZipUtils.getBinaryContent(`someURL/${file}`, async (err, data) => {
try {
if (err) throw err;
const zip = await JSZip.loadAsync(data);
const name = file.replace(".zip", "");
resolve(await zip.file(name).async('text'));
} catch(error) {
reject(error);
}
});
});
}
<script>/*IGNORE*/const JSZipUtils = {getBinaryContent:(p,c)=>errs.gbc?c(new Error('gbc'),null):c(null,{foo:true})};const JSZip = {loadAsync:(d)=>errs.la?Promise.reject(new Error('la')):({file:n=>({async:a=>errs.a?Promise.reject(new Error('a')):Promise.resolve('Hello World')})})};const errs = {gbc:false,la:false,a:false};/*IGNORE*/</script>

This kind of sounds like a use case for async iterator generators, but maybe I'm just over-engineering. You have a bunch of resources that you want to iterate over and their contents are asynchronous. You want it to "look" synchronous, so you can leverage async/await:
function getZip(zipFile) {
/*
* Theres no point in simplifying this function since it looks like
* the JSZip API deals with callbacks and not Promises.
*/
return Promise.resolve(zipFile);
}
function someFn(a, b) {
return `${a}: ${b.length}`;
}
async function* zipper(elements) {
for (const element of elements) {
for (const data of element) {
const txt = await getZip(data);
yield someFn(txt, data);
}
}
}
(async() => {
const elements = [
["hello"],
["world"],
["foo"],
["bar"]
];
let total = [];
for await (const out of zipper(elements)) {
total.push(out);
}
console.log(total);
})();

Enqueue callback functions then await their execution

I want to create a function that enqueues callback functions, but doesn't return until that callback has been dequeued and executed.
function executeCallback(callback) {
if( shouldEnqueue ) {
enqueue(callback);
// awaits execution of callback
// return executed callback's return value
} else {
return callback()
}
}
The actual function works as follows. When an orderbook is needed, we will check to see if we have a recent one cached. If we don't have a recent one, we will queue the request for a fresh orderbook. We will then return the orderbook once it has been requested. The actual code will look something like this:
const axios = require("axios");
const moment = require("moment");
const { promisify } = require("util");
const orderbookQueue = [];
const orderbookUpdateTime = {};
async function requestOrderbook(pair) {
const response = await axios.post("https://api.kraken.com/0/public/Depth", {
pair
});
orderbookUpdateTime[pair] = +moment();
return response.data.result[pair];
}
async function getOrderbook(pair, currentTime) {
if (
orderbookUpdateTime[pair] &&
currentTime - orderbookUpdateTime[pair] > 60000
) {
const requestOrderbookAsync = promisify(requestOrderbook);
orderbookQueue.push(() => requestOrderbookAsync(pair));
// how to I return the result of requestOrderbookAsync when it has been called ?
}
return requestOrderbook(pair);
}
const queueExecutor = function queueExecutor() {
while (orderbookQueue.length) {
orderbookQueue[0]();
orderbookQueue.shift();
}
};

1. Convert you callbacks to promise
E.g, convert
function mutilpy(a, b, callback) {
let result = a*b;
return callback(result);
}
to
function multiply(a, b) {
return new Promise(function(resolve, reject) {
let result = a*b;
resolve(result);
})
}
2. (Sort of) Enqueue your callbacks for completion
var promise1 = Promise.resolve(3);
var promise2 = 42;
var promise3 = multiply(10,3);
Promise.all([promise1, promise2, promise3]).then(function(values) {
// YOU GET HERE WHEN ALL PROMISES ARE RESOLVED
console.log(values);
});
// expected output: Array [3, 42, 30]

I found a solution that works for me. One can extract the resolve function from a new Promise, enqueue the resolve function in a different scope, then dequeue and execute the resolve function whenever the time comes, in any scope.
Resolve Javascript Promise outside function scope

How to correctly set async recursion in node.js?

Here is one example function.
async function getRandomBig() {
let result;
result = await randomModule.getRandom();
if (result > 0.9 ) {
return getRandomBig();
} else {
return result;
}
}
So Obviously, I would like that execution to randomModule.getRandom() happens asynchronously. Is this example the way to go?
Also, I would like to know how to make the first call to getOutput be asynchronous.
Thank you

It's a little hard to answer your question given the info you've provided, but maybe this will help:
The async function will return a promise to the outside caller. Than means that results obtained from await will happen asynchronously. For example consider this code:
// normal synchronous function
function changeRes() {
return "after"
}
// sync function that `awaits` it's result
async function getget() {
res = await changeRes()
}
let res = "before"
// p will be a pending promise
var p = getget()
// what is res at this point?
console.log(res) // still "before" because await doesn't return until next tick
// now res is 'after'
p.then(() => console.log(res))
But be careful because the call to changeRes is not asynchronous — it is called in the current loop. Compare this with the first code. We only change the behavior of changeRes():
function changeRes() {
res = "after"
return res
}
async function getget() {
res = await changeRes()
}
let res = "before"
// p is still a pending promise
var p = getget()
// but res was changed in changeRes synchronously
console.log(res)
p.then(() => console.log(res))
EDIT based on comment:
With the recursive function, everything important is happening inside the async function, so it all should work as expected. For example, you can substitute your randomModule.getRandom with the regular Math.random() which is synchronous, but the await will make it work in the context of an async function. So this function will return a promise that resolves to a random float less than 0.25:
async function getRandomBig() {
let result;
result = await Math.random();
if (result > 0.25) {
return getRandomBig();
} else {
return result;
}
}
getRandomBig().then(console.log)
So will this even though it's truly async:
function asyncRandom(){
return new Promise(resolve => {
setTimeout(() => resolve(Math.random()), 500)
})
}
async function getRandomBig() {
let result;
result = await asyncRandom();
if (result > 0.25 ) {
console.log("too big recurse")
return getRandomBig();
} else {
return result;
}
}
getRandomBig().then(console.log)

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);
});

Bluebird Promising the result of a heavy function

I've been using Bluebird a lot recently on a HAPI API development. I've just run into my first real problem, that perhaps my understanding or naivety has me stumped.
The following code is an example of what I am facing:-
var Promise = require('bluebird'),
stuff = require('../stuff');
module.exports = {
getSomething: function(request, reply) {
var p = Promise.resolve();
p = p.then(function() {
return db.find() //etc. etc.
});
p = p.then(function(resultFromPromise) {
//problems begin here
var data = stuff.doSomeReallyLongAndBoringFunction(resultFromPromise);
return data;
});
p.then(function(data) {
//no data here.
});
};
};
I've commented where the problems usually begin. the stuff.doSomeReallyLongAndBoringFunction() returns an object (using more promises concidently) and it's this object I want to access, but //no data here always fires before data returns. stuff.doSomeReallyLongAndBoringFunction() continues to run regardless and completes successfully, but after the code goes async, I don't know how to promise that function's return value back.
Can anyone offer any guidance? Please accept my apologies for any naivety in the question!
Help as always, is appreciated
NB just for clarity, stuff.doSomeReallyLongAndBoringFunction() does not return a Promise itself. Although, I did try return new Promise(reject, resolve) { }); manual wrap. It is simply a function that uses promises itself (successfully) to get data.
Update 1
stuff.doSomeReallyLongAndBoringFunction() is too big to post directly, but it does something like this:-
var Promise = require('bluebird'),
rp = require('request-promise');
module.exports = {
doSomeReallyLongAndBoringFunction: function() {
var p = Promise.resolve();
p = p.then(function() {
return db.find() //etc. etc.
});
p.then(function() {
rp(options).then(function(response){
//get some data from remote location
}).then(function(dataFromService) {
//do some jiggery pokery with said data
var marshalledData = dataFromService;
db.something.create({
Field: 'something'
}).exec(function(err, saved) {
return marshalledData;
});
});
}).catch(function(err) {
});
};
};
Update 2
Thank you Justin for your help. Here is the actual code, perhaps this may help?
Promise.resolve()
.then(function() {
if(typeof utils.intTryParse(place) !== 'number') {
return foursquare.createPlaceFromFoursquare(sso, place, request, reply);
} else {
return { Place: { PlaceId: place }};
}
}).then(function(placeObj) {
console.log('Place set as', placeObj); //always returns undefined, despite function actually completing after async op...
});

If your doSomeReallyLongAndBoringFunction is really running asynchronously, then it doesn't make sense to run it the way you have setup.
Edit - Here's a simple explanation of the way your code looks to be running vs a refactored version. It's been simplified , so you'll need to fill in the relevant sections with your actual implementation.
var Promise = require('bluebird');
function myAsync() {
setTimeout(function(){
return 'done sleeping';
}, 2000);
};
//The way your code is running
Promise.resolve()
.then(function(){
return 'hello';
})
.then(function(done){
console.log(done);
return myAsync(); //your error is here
})
.then(function(done){
console.log(done);
});
//refactored
Promise.resolve()
.then(function(){
return 'hello';
})
.then(function(done){
console.log(done);
return new Promise(function(resolve) {
setTimeout(function(){
resolve('done sleeping');
}, 2000);
});
})
.then(function(done){
console.log(done);
});

just for clarity, stuff.doSomeReallyLongAndBoringFunction() does not return a Promise itself.
And that's your problem. As it does something asynchronous and you want to get its result, it should return a promise. In fact, that's the case for every asynchronous function, especially then callbacks! It should be something like
module.exports = {
doSomeReallyLongAndBoringFunction: function() {
return db.find()
// ^^^^^^
.then(function() {
return rp(options).then(function(response){
// ^^^^^^
//get some data from remote location
}).then(function(dataFromService) {
//do some jiggery pokery with said data
var marshalledData = dataFromService;
return db.something.create({
// ^^^^^^
Field: 'something'
}).execAsyc();
});
}).catch(function(err) {
});
}
};
Your getSomething method has the same issues, and should look like this:
var createPlace = Promise.promisify(foursquare.createPlaceFromFoursquare);
module.exports = {
getSomething: function(request) {
var p;
if (typeof utils.intTryParse(place) !== 'number')
p = createPlace(sso, place, request); // this returns a promise!
else
p = Promise.resolve({Place: {PlaceId: place}});
return p.then(function(placeObj) {
// ^^^^^^
console.log('Place set as', placeObj);
});
}
};
See also these generic rules for promise development.

doSomeReallyLongAndBoringFunction needs to look like this:
doSomeReallyLongAndBoringFunction: function(param) {
var resolver = Promise.defer();
/*
* do some asynchronous task and when you are finished
* in the callback, do this:
*/
resolver.resolve(resultFromAsyncTask);
/*
*
*
*/
return resolver.promise;
}