Develop Reference

Promise.all after a foreach loop

I'm trying to use Promise.all() with an array of Promises that is being populated inside of a foreach looop right before but it seems like the Promise.all() is not waiting the promises to be all completed before executing its callback.
What's wrong with the following code? (I tried to simplify it before posting, so parts of it might not make complete sense, but that promised and loops are all there).
class test {
constructor(sql) {
Promise.all([this.sync(sql, 0), this.sync(sql, 1)]).then((data) => {
console.log(data);
});
}
sync(sql, id = 0) {
return new Promise((resolve, reject) => {
request.get('http://localhost/test/' + id, {
json: true
}, (req, res) => {
var promises = [];
res.body['items'].forEach(item => {
promises.push(new Promise((resolve, reject) => {
this.existingRecord(sql, item['id']).then(() => {
resolve(false);
}).catch(() => {
this.add(sql, item).then(resolve(id));
})
}))
});
Promise.all(promises).then((data) => resolve(data));
});
});
}
add(sql, data) {
return new Promise((resolve, reject) => {
console.log('Inserting ' + data['id']);
var request = new sql.Request();
var query = `INSERT INTO test (col1, col2) VALUES (${utils.prepareInsertdata(data)})`;
request.query(query, (err, result) => {
if (err) {
console.log('ERROR INSERTING: ' + data['id']);
console.log(err);
}
resolve();
});
});
}
}

First off, you're making it much harder to write good, clean, error-free code when you have a mix of promises and regular callbacks in your control flow. I find that the best way to write asynchronous code using promises is to first take any asynchronous operations that are not promise based and create promise-based wrappers for them and then write my logic and control flow using only promises. This makes a consistent path for flow of control and for error handling and it removes the mess of promisifying things from the actual main logic.
Then, I see several significant issues in your code.
Asynchronous operations in the constructor
It's almost never a good idea to put asynchronous operations in the constructor. This is because the constructor HAS to return the object itself so that leaves no simple way to communicate back to the code that created your object when the asynchronous operations are actually done and if they succeeded of failed. It is not entirely clear to me what you're trying to accomplish with those async operations, but this is likely a bad design pattern. I favor a factory function that returns a promise that resolves to the new object for combining the creation of an object with asynchronous operations. This gives you everything you need, a fully formed object, knowledge of when the async operations are done and an ability to have error handling for the async operations. You can see more about this factory function option and some other design options here:
Asynchronous operations in constructor
Improve .then() handler construction
When you do this:
this.add(sql, item).then(resolve(id));
You are calling resolve(id) immediately and passing that to .then() rather than waiting for the .then() handler to be called before calling resolve(id). All of this is complicated because you're mixing regular callbacks and promises.
Creating new wrapped promises rather than just returning existing promises
This is related to your mix of regular callbacks and regular promises, but you'd much rather just return an existing promise than wrap it in a new promise that you have to manually resolve and reject. More than half the time, you will miss proper error handling when manually wrapping things in a new promise and it just results in more code than is needed.
Race Conditions
In any sort of multi-user database environment, you can't write database code such as:
if (record exists) {
do one thing
} else {
create new record
}
This is a race condition. If some other database request comes in during the processing of this, it could change the database in the middle of this and you'd be trying to create a record that just got created by another piece of code.
The usual solution varies by database (and you don't say exactly which database library you're using). Usually, you want to let the database manage the creation of unique records making it so that a duplicate record (by whatever key you're managing uniqueness in this table by) isn't allowed in the database and the concurrency of that is managed by the database itself. Some databases have an atomic operation such as findOrCreate() that will find an existing record or create a new one in an atomic fashion. Other databases have other approaches. But, it's important to make sure that adding unique records to the database is an atomic operation that can't ever create unwanted duplicates.
I'd suggest this implementation:
// use promise version of request library (already promisified for us)
const rp = require('request-promise');
class test {
constructor() {
}
init(sql) {
return Promise.all([this.sync(sql, 0), this.sync(sql, 1)]).then((data) => {
console.log(data);
// do something with the data here - probably store it in instance data
});
}
sync(sql, id = 0) {
return rp.get('http://localhost/test/' + id, {json: true}).then(res => {
// process all items
return Promise.all(res.body.items.map(item => {
return this.existingRecord(sql, item.id).then(() => {
return false;
}).catch(() => {
// it's probably bad form here to do this for all possible database errors
// probably this should be looking for a specific error of id not found
// or something like that.
// This is also likely a race condition. You would typically avoid the race
// condition by making the item key unique in the database and just doing an add and letting
// the database tell you the add failed because the item already exists
// This will allow the database to control the concurrency and avoid race conditions
return this.add(sql, item);
});
}));
});
}
}
// factory function that returns promise that resolves to a new object
// don't use new test() elsewhere
function createTestObj(sql) {
let t = new test();
return t.init(sql).then(() => {
// resolve to our new object
return t;
});
}
For your add() method, I'd switch to using the promise interface in your sql database. There should either be one built-in or a 3rd party package that will add one on top of your database interface. This will prevent the manual creation of promises and the incomplete error handling in your add() method.

Loop through AWS Lambda Nodejs SDK function

I'm new to Nodejs and having trouble understand this issue: I tried to run a describe function against an array, and the AWS function seems to run after the main function has finished.
Here's the main function: (loop thru a list of ACM ARNs and check the status)
var checkCertStatus = function(resolveObj){
var promise = new Promise(function(resolve, reject){
console.log('1');
var retObj='';
resolveObj.Items.forEach(function(element) {
var certDescribeParams = {
CertificateArn: element.sslCertId
};
console.log('2');
acm.describeCertificate(certDescribeParams, function(err, data) {
if(err) reject(new Error(err));
else {
console.log(data.Certificate.DomainName + ': ' + data.Certificate.Status);
retObj+=data;
}
});
});
console.log('3');
resolve(retObj);
return promise;
})
}
Based on the debug log, assuming there are 2 items need to be processed, what I got:
1
2
2
3
example.com: ISSUED
example2.com: ISSUED
Basically, I need to pass this result to the next function in the chain (with promise and stuff).

Welcome to Node.js! Speaking generally, it might be helpful to study up on the asynchronous programming style. In particular, you seem to be mixing Promises and callbacks, which may make this example more confusing than it needs to be. I suggest using the AWS SDK's built-in feature to convert responses to Promises.
The first thing I notice is that you are manually constructing a Promise with a resolve/reject function. This is often a red flag unless you are creating a library. Most other libraries support Promises which you can simply use and chain. (This includes AWS SDK, as mentioned above.)
The second thing I notice is that your checkCertStatus function does not return anything. It creates a Promise but does not return it at the end. Your return promise; line is actually inside the callback function used to create the Promise.
Personally, when working with Promises, I prefer to use the Bluebird library. It provides more fully-featured Promises than native, including methods such as map. Conveniently, the AWS SDK can be configured to work with an alternative Promise constructor via AWS.config.setPromisesDependency() as documented here.
To simplify your logic, you might try something along these lines (untested code):
const Promise = require('bluebird');
AWS.config.setPromisesDependency(Promise);
const checkCertStatus = (resolveObj) => {
const items = resolveObj.Items;
console.log(`Mapping ${items.length} item(s)`);
return Promise.resolve(items)
.map((item) => {
const certDescribeParams = {
CertificateArn: item.sslCertId,
};
console.log(`Calling describeCertificate for ${item.sslCertId}`);
return acm.describeCertificate(certDescribeParams)
.promise()
.then((data) => {
console.log(`${data.Certificate.DomainName}: ${data.Certificate.Status}`);
return data;
});
});
};
We're defining checkCertStatus as a function which takes in resolveObj and returns a Promise chain starting from resolveObj.Items. (I apologize if you are not yet familiar with Arrow Functions.) The first and only step in this chain is to map the items array to a new array of Promises returned from the acm.describeCertificate method. If any one of these individual Promises fails, the top-level Promise chain will reject as well. Otherwise, the top-level Promise chain will resolve to an array of the results. (Note that I included an inessential .then step just to log the individual results, but you could remove that clause entirely.)
Hope this helps, and I apologize if I left any mistakes in the code.

Node.js async, but only handle first positive/defined result

What is the best way to create parallel asynchronous HTTP requests and take the first result that comes back positive? I am familiar with the async library for JavaScript and would happy to use that but am not sure if it has exactly what I want.
Background - I have a Redis store that serves as state for a server. There is an API we can call to get some data that takes much longer than reaching the Redis store.
In most cases the data will already be in the Redis store, but in some cases it won't be there yet and we need to retrieve it from the API.
The simple thing to do would be to query Redis, and if the value is not in Redis then go to the API afterwards. However, we'll needlessly lose 20-50ms if the data is not yet in our Redis cache and we have to go to the API after failing to find the data with Redis. Since this particular API server is not under great load, it won't really hurt to go to the API simultaneously/in parallel, even if we don't absolutely need the returned value.
//pseudocode below
async.minimum([
function apiRequest(cb){
request(opts,function(err,response,body){
cb(err,body.result.hit);
}
},
function redisRequest(cb){
client.get("some_key", function(err, reply) {
cb(err,reply.result.hit);
});
}],
function minimumCompleted(err,result){
// this mimimumCompleted final callback function will be only fired once,
// and would be fired by one of the above functions -
// whichever one *first* returned a defined value for result.hit
});
is there a way to get what I am looking for with the async library or perhaps promises, or should I implement something myself?

Use Promise.any([ap, bp]).
The following is a possible way to do it without promises. It is untested but should meet the requirements.
To meet requirement of returning the first success and not just the first completion, I keep a count of the number of completions expected so that if an error occurs it can be ignored it unless it is the last error.
function asyncMinimum(a, cb) {
var triggered = false;
var completions = a.length;
function callback(err, data) {
completions--;
if (err && completions !== 0) return;
if (triggered) return;
triggered = true;
return cb(err, data);
}
a.map(function (f) { return f(callback); });
}
asyncMinimum([
function apiRequest(cb){
request(opts,function(err,response,body){
cb(err,body.result.hit);
}
},
function redisRequest(cb){
client.get("some_key", function(err, reply) {
cb(err,reply.result.hit);
});
}],
function minimumCompleted(err,result){
// this mimimumCompleted final callback function will be only fired once,
// and would be fired by one of the above functions -
// whichever one had a value for body.result.hit that was defined
});

The async.js library (and even promises) keep track of the number of asynchronous operations pending by using a counter. You can see a simple implementation of the idea in an answer to this related question: Coordinating parallel execution in node.js
We can use the same concept to implement the minimum function you want. Only, instead of waiting for the counter to count all responses before triggering a final callback, we deliberately trigger the final callback on the first response and ignore all other responses:
// IMHO, "first" is a better name than "minimum":
function first (async_functions, callback) {
var called_back = false;
var cb = function () {
if (!called_back) {
called_back = true; // block all other responses
callback.apply(null,arguments)
}
}
for (var i=0;i<async_functions.length;i++) {
async_functions[i](cb);
}
}
Using it would be as simple as:
first([apiRequest,redisRequest],function(err,result){
// ...
});

Here's an approach using promises. It takes a little extra custom code because of the non-standard result you're looking for. You aren't just looking for the first one to not return an error, but you're looking for the first one that has a specific type of result so that takes a custom result checker function. And, if none get a result, then we need to communicate that back to the caller by rejecting the promise too. Here's the code:
function firstHit() {
return new Promise(function(resolve, reject) {
var missCntr = 0, missQty = 2;
function checkResult(err, val) {
if (err || !val) {
// see if all requests failed
++missCntr;
if (missCntr === missQty) {
reject();
}
} else {
resolve(val);
}
}
request(opts,function(err, response, body){
checkResult(err, body.result.hit);
}
client.get("some_key", function(err, reply) {
checkResult(err, reply.result.hit);
});
});
}
firstHit().then(function(hit) {
// one of them succeeded here
}, function() {
// neither succeeded here
});
The first promise to call resolve() will trigger the .then() handler. If both fail to get a hit, then it will reject the promise.

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

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

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

Using Javascript native promises, resolve a promise after thenables attached

Is there a way using javascript native promises (docs) to create a promise and attach thenables, without knowing at constructor time how it will resolve?
var foo = new Promise(function(resolve, reject) {
// I don't know how this will resolve yet as some other object will resolve it
});
foo.then(function(val) {
console.log("first " + val);
});
foo.resolve("bar");
foo.then(function(val) {
console.log("second " + val);
});
// result
// first bar
// second bar

Simply save them inside of a closure.
var makePromise = function () {
var resolvePromise = null,
rejectPromise = null,
promise = new Promise(function (resolve, reject) {
resolvePromise = resolve;
rejectPromise = reject;
});
return { promise : promise, resolve : resolvePromise, reject : rejectPromise };
};
var deferredSomething = function () {
var deferredThing = makePromise();
waitAWhile()
.then(doStuff)
.then(function (result) {
if (result.isGood) {
deferredThing.resolve(result.data);
} else {
deferredThing.reject(result.error);
}
});
return deferredThing.promise;
};
This is actually the majority of the difference between the "deferred" concept and the "promise" concept; one more level, on top, that has the actual remote-controls that you can give to someone else, while you hand the .then|.success|.done|etc... to your consumers.
Once you bring those functions out into your upstream process, you can happily lazy-load whatever you'd like, using the "thenable" which you'll return, and then succeed or fail your chain (or leave it hanging) at will...
UPDATE
Seeing as this is probably going to continue to be the chosen answer, and continue to be voted down, as the solution to the exact problem he was having (ie: retrofitting code which was not made with ES6 promises in mind), I figure I'll add a more detailed example of exactly why using this antipattern selectively can be better than naught:
MongoClient.connect("mongodb://localhost:21017/mydb", (err, db) => {
db.collection("mycollection", (err, collection) => {
collection.find().toArray((err, results) => {
doStuff(results);
});
});
});
If I were to write a library, here, hoping to reach the point where I could write:
let dbConnected = MongoClient.connect(dbURL);
dbConnected
.then(db => db.collection(myCollection))
.then(collection => collection.find(query))
.then(stream => doStuff(stream));
...or alternatively:
composeAsync(
(stream) => doStuff(stream),
(collection) => collection.find(query),
(db) => dbCollection(myCollection)
)(dbConnected);
...for ease of use within the library, does it make sense to wrap every single function-body within an instantiated promise
// find = curry(query, collection)
return new Promise(resolve, reject) {
/* whole function body, here /
/ do lots of stuff which is irrelevant to the resolution of mongo.db.collection.find, but is relevant to its calling */
collection.find(query).toArray( /node-callback/(err, result) {
if (err) {
reject(err);
} else {
resolve(result);
}
});
};
...or in looking at the pattern of really only requiring the node-specific callback to be resolved, does it make more sense to have some form of promise-resolver, to save having to write out / copy-paste a half-dozen purely-redundant lines which should be completely DRYed up?
// find = curry(query, collection)
let resolver = new NodeResolver();
collection.find(query).toArray(promise.resolve);
return resolver.promise;
Yes, that is an anti-pattern... ...yet, an antipattern which requires fewer keystrokes, restores the natural flow of the promise-chain, fixes a problem with Node's callback-only API, reduces the potential for errors, et cetera.
Yes, there are already libraries which do this...
...solutions specific to X library or Y...
...or solutions which globally override methods of various modules (scary)
...or solutions which, again, basically force you to pass in all of the details of the call you're making:
wrapNodeMethod(fs, "read", url, config).then(data => { /*...*/ });
But there is no simple solution for the case of inverting all of that pain, without either:
a) wrapping the entire function body in a promise, to feed the async callback a resolver
b) using an antipattern within a library, in order to pass Node callbacks a resolver that the rest of the function-body needs to know precisely nothing about.
Even if data needed to be transformed within the resolver, it would still make more sense to return a transformed set, in a new promise
let resolver = new NodeResolver();
somethingAsync(resolver.resolve);
return resolver.promise.then(transformData).then(logTransform);
...than to wrap the whole body, including transforms, etc, just for closure-scope reference, just for the sake of avoiding an "antipattern" which clearly goes against the grain of what has become a very prominent JS platform/paradigm.
Now, personally, I'd be happier if IO||Node methods returned a promise and/or a stream, as well as taking a callback, as a core part of the platform...
...that's not going to happen...
...but you can't possibly tell me that writing less, and keeping Node modules DRY, while still using ES6 Promises is an "antipattern", without providing me a more-eloquent solution, therefore.
If you can, indeed, provide me something that I can use in any NodeJS callback, which does, indeed, provide a more eloquent solution to this, such that I don't have to wrap every body of every method which contains an async callback in a new constructor, or use clunky dispatcher methods, or hijack entire modules to override their global functionality...
...I'd be more than willing to retract my statement that this particular pattern is still highly-useful, as regards interfacing with APIs which are prone to pyramids o'dewm.

If result of promise depends on other promise, you should just create a promise using then.
What was proposed by #Norguard in direct form, doesn't make much sense (it's even coined as deferred anti-pattern). Below code does exactly same, and no extra promise is needed:
var deferredSomething = function () {
return waitAWhile()
.then(doStuff)
.then(function (result) {
if (result.isGood) {
return result.data;
} else {
throw result.error;
}
});
});
};
And, even if, for whatever reason you would need to create promise upfront, then with constructor pattern it would be cleaner to do it that way:
var deferredSomething = function () {
return new Promise(function (resolve, reject) {
waitAWhile()
.then(doStuff)
.then(function (result) {
if (result.isGood) {
resolve(result.data);
} else {
reject(result.error);
}
});
});
};