We Keep Coding

How to wrap an async package function?

I'm using the popular node library, got, to make simple GET requests to a JSON API.
I have a function that abstracts the request, like so:
function performRequest(url) {
got(url, {
json: true
}).then(function (response) {
return formatResponse(response.body);
}).catch(function (error) {
console.log(error.response.body);
});
}
formatResponse is a simple synchronous method that modifies the JSON returned from the API.
I would like to be able to call performRequest from another function and then use the return value (once resolved). Currently, as performRequest is not recognized as an async method, my code is calling it and then proceeding immediately.
function myBigFunction() {
var url = composeUrl();
var res = performRequest(url);
doMoreStuffWithResponse(res);
}
I know that I need to utilize a Promise, however, I'm always unclear as to how to use a Promise in conjunction with a built-in library function that is already using a Promise (like in this case).
I'm also completely open to the possibility that I'm going about this all wrong. In that case, I would appreciate some redirection.
Thank you for your time.

Understand what a Promise is. Its a value, you can treat it as such. In order to "read" the value, you pass a function to the Promise's then method. You don't need myBigFunction. Anything you want to run after the Promise resolves just needs to be passed to then:
var req = performRequest(composeURL());
req.then(doStuffWithResponse);
Now, I don't particularly care for this way although I do it fairly often. I prefer to have functions that take promises and invoke their then method:
var takesAPromise = function(p) {
return p.then(/* does stuff */);
};
Note that it returns the Promise of the completed task. But what I like even better is this ES6 one-liner:
let wrap = f => p => p.then(val => f.call(null, val));
Now you can wrap arbitrary functions to take Promises as input and return them as output. If Promises were a monad, this would be their bind function. Making it work seamlessly with functions of arbitrary arity is left as an exercise to the reader.

You'll always want to return a promise from your functions:
function performRequest(url) {
return got(url, {
//^^^^^^
json: true
}).then(function(response) {
return formatResponse(response.body);
}, function(error) {
throw new Error(error.response.body);
});
}
With this, you can wait for the result in your big functions using another then:
function myBigFunction() {
var url = composeUrl();
var promise = performRequest(url);
return promise.then(function(res) {
return doMoreStuffWithResponse(res);
});
}
or in short
function myBigFunction() {
return performRequest(composeUrl()).then(doMoreStuffWithResponse);
}
so that you can call it like
myBigFunction().catch(function(error) {
console.log(error.message);
});

Javascript: Utilising data from an async callback function (MongoClient)

I've spent so much time trying to find the answer to this on here and come up with nothing. Hoping someone can enlighten me..
I have code which is making an async call to a database and returning data in a callback function (in my case I'm using MongoClient, which returns a Promise). However, I can't work out how to use the resulting data to actually set function-level variables - whenever I try to do it the resulting value that I log is either undefined or a pending Promise object.
There's lots of posts on this subject but I can't find any methods that work when I try to apply them. Any and all help gratefully received!
function lookupOneDbEntry(key, value) {
var responseData = "initial data"
// search for the entry in the database
db.collection("my_collection").findOne({key: value}, function(err, result) {
if (err) {
//if database throws an error
responseData = "db error";
}
else {
// if the entry is found, return the data
responseData = result;
}
});
return responseData;
}
EDIT: I am aware of other posts on this (like this one here and, while exhaustive documentation is useful to an extent, I;m having trouble using this information practically in a real-life implementation like the one above. Hence my question here.

Async calls happens outside of the call stack that you are on. you can't return it onto the current stack.
So we use the promises to hook into the results of our call.
function lookupOneDbEntry(key, value) {
return new Promise(function (resolve, reject) {
// search for the entry in the database
db.collection("my_collection").findOne({key: value}, function(err, result) {
if (err) {
//if database throws an error
reject(err);
}
else {
// if the entry is found, return the data
resolve(result);
}
});
});
}
lockupOneDbEntry('myKey', 'myValue').then(function (result) {
console.log('result', result);
}, function (err) {
console.log("error!", err);
});

After a long while of experimenting I've finally managed to do it - I didn't need any fancy callbacks or additional Promises in the end, I just removed the optional callback in the database request and instead processed the returned promise separately.
function lookupOneDbEntry(key, value) {
var responseData = "initial data";
var solution = db.collection("accounting_module").findOne({key: value});
solution.then(function (result) {
// validation of result here
responseData = result;
});
return responseData;
}

how does the callback in async.parallel work (where does it come from?)

I found this example in the docs. I understand how the main callback (function (err,results)...) works but what is the 'callback' parameter that is passed into one and two functions? Where are those coming from and what do they do?
async.parallel({
one: function(callback){
setTimeout(function(){
callback(null, 1);
}, 200);
},
two: function(callback){
setTimeout(function(){
callback(null, 2);
}, 100);
}
},
function(err, results) {
// results is now equals to: {one: 1, two: 2}
});

The callback argument is passed to your functions by the async infrastructure. It points to a function inside of async (though that's not something you need to know). The async infrastructure passes it to you and asks you to call it at the right time.
This is how you communicate back to the async library to tell it that your async function has finished its work and also whether it finished successfully or with an error. When your function finishes it's work, you call that function and that notifies the async library that this step of the process is now done (with either error or success).
FYI, if you're familiar with Express middleware in node.js, it's very similar to the next argument that is passed to middleware. When your middleware gets called, one of the arguments passed to it is the next callback. Your middleware does its work (which could be async) and then when it's done, it tells the Express infrastructure that it's done by calling the next() callback that was passed to it.
Here's a somewhat similar example from Express:
app.use(function (req, res, next) {
doSomethingAsync(function() {
next();
})
});

It doesn't come from anywhere, its just how the library works, a callback is used to pass data to the next function.
This is the function here:
function _parallel(eachfn, tasks, callback) {
callback = callback || noop;
var results = isArrayLike(tasks) ? [] : {};
eachfn(tasks, function (task, key, callback) {
task(rest(function (err, args) {
if (args.length <= 1) {
args = args[0];
}
results[key] = args;
callback(err);
}));
}, function (err) {
callback(err, results);
});
}
https://github.com/caolan/async/blob/master/dist/async.js#L3471

How do I wait for a promise to finish before returning the variable of a function?

I'm still struggling with promises, but making some progress thanks to the community here.
I have a simple JS function which queries a Parse database. It's supposed to return the array of results, but obviously due to the asynchronous nature of the query (hence the promises), the function returns before the results, leaving me with an undefined array.
What do I need to do to make this function wait for the result of the promise?
Here's my code:
function resultsByName(name)
{
var Card = Parse.Object.extend("Card");
var query = new Parse.Query(Card);
query.equalTo("name", name.toString());
var resultsArray = [];
var promise = query.find({
success: function(results) {
// results is an array of Parse.Object.
console.log(results);
//resultsArray = results;
return results;
},
error: function(error) {
// error is an instance of Parse.Error.
console.log("Error");
}
});
}

Instead of returning a resultsArray you return a promise for a results array and then then that on the call site - this has the added benefit of the caller knowing the function is performing asynchronous I/O. Coding concurrency in JavaScript is based on that - you might want to read this question to get a broader idea:
function resultsByName(name)
{
var Card = Parse.Object.extend("Card");
var query = new Parse.Query(Card);
query.equalTo("name", name.toString());
var resultsArray = [];
return query.find({});
}
// later
resultsByName("Some Name").then(function(results){
// access results here by chaining to the returned promise
});
You can see more examples of using parse promises with queries in Parse's own blog post about it.

What do I need to do to make this function wait for the result of the promise?
Use async/await (NOT Part of ECMA6, but
available for Chrome, Edge, Firefox and Safari since end of 2017, see canIuse)
MDN
async function waitForPromise() {
// let result = await any Promise, like:
let result = await Promise.resolve('this is a sample promise');
}
Added due to comment:
An async function always returns a Promise, and in TypeScript it would look like:
async function waitForPromise() {
// let result = await any Promise, like:
let result: Promise<string> = await Promise.resolve('this is a sample promise');
}

You're not actually using promises here. Parse lets you use callbacks or promises; your choice.
To use promises, do the following:
query.find().then(function() {
console.log("success!");
}, function() {
console.log("error");
});
Now, to execute stuff after the promise is complete, you can just execute it inside the promise callback inside the then() call. So far this would be exactly the same as regular callbacks.
To actually make good use of promises is when you chain them, like this:
query.find().then(function() {
console.log("success!");
return new Parse.Query(Obj).get("sOmE_oBjEcT");
}, function() {
console.log("error");
}).then(function() {
console.log("success on second callback!");
}, function() {
console.log("error on second callback");
});

You don't want to make the function wait, because JavaScript is intended to be non-blocking.
Rather return the promise at the end of the function, then the calling function can use the promise to get the server response.
var promise = query.find();
return promise;
//Or return query.find();

Call async function while pushing the same function on array

I have this code:
var queue = [];
var allParserd = [];
_.each(webs, function (web) {
queue.push(function () {
WebsitesUtils.parseWebsite(web, function (err, parsed) {
allParserd.push(parsed);
});
});
});
Promise.all(queue).then(function (data) {
console.log(allParserd);
});
Basically I need to fetch all my webs and be sure to give the result after that every parsing is done. the function parseWebsite return the correct data, but in this way is not called and allParsed return just as an empty array. I'm sure that I miss some things, I've started to use the promises just from some days.
If you need some more information just tell me.
P.s.
I want that all the functions to start at the same time; I don't want to wait for each one response for going forward.

Tagged with Bluebird so let's use it:
First, let's convert your callback API to promises:
Promise.promisifyAll(WebsitesUtils);
Now, let's use .map to map every item in webs to it being parsed parseWebsite:
Promise.map(webs, function(item){
return WebsitesUtils.parseWebsiteAsync(item); // note the suffix
}).then(function(results){
// all the results are here.
}).catch(function(err){
// handle any errors
});
As you can see - this is trivial to do with Bluebird.

Promise.all doesn't take a queue of functions to execute. It expects an array of promises which represent the results of the many concurrently running (still pending) requests.
The first step is to have a function that actually returns a promise, instead of only executing a callback. We can use
function parseWebsite(web) {
return new Promise(function(fulfill, reject) {
WebsitesUtils.parseWebsite(web, function (err, parsed) {
if (err)
reject(err);
else
fulfill(parsed);
});
});
}
or simply use promisification that does this generically:
var parseWebsite = Promise.promisify(WebsitesUtils.parseWebsite, WebsitesUtils);
Now we can go to construct our array of promises by calling that function for each site:
var promises = [];
_.each(webs, function (web) {
promises.push(parseWebsite(web));
});
or just
var promises = _.map(webs, parseWebsite);
so that in the end we can use Promise.all, and get back our allParsed array (which even is in the same order as webs was!):
Promise.all(promises).then(function(allParsed) {
console.log(allParsed);
});
Bluebird even provides a shortcut function so you don't need promises:
Promise.map(webs, parseWebsite).then(function(allParsed) {
console.log(allParsed);
});

Here's how might do it with async:
var async = require('async');
var webs = ...
async.map(webs, function(web, callback) {
WebsitesUtils.parseWebsite(web, callback);
}, function(err, results) {
if (err) throw err; // TODO: handle errors better
// `results` contains all parsed results
});
and if parseWebsite() isn't a prototype method dependent on WebsitesUtils then you could simplify it further:
async.map(webs, WebsitesUtils.parseWebsite, function(err, results) {
if (err) throw err; // TODO: handle errors better
// `results` contains all parsed results
});