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readFile returns undefined

I have this situation where I'm trying to send a request with the content of a file, but the problem is that the content arrives undefined. How can I solve this? I tried multiple versions from stackoverflow, but nothing worked so far.
const ifExists = (filePath) => {
try {
if (fs.existsSync(filePath)) {
return true;
}
} catch (err) {
console.log(err);
}
return false;
}
const readMyFile = async (filePath) => {
const fileExists = ifExists(filePath);
if (fileExists) {
fs.readFile(filePath, (err, data) => {
if (err) {
console.log("Error occurred when trying to read the file.");
return false;
}
console.log("File successfully read.");
return data; // data has the right content here
});
} else {
console.log("File not found");
return false;
}
}
const getFile = async function (req, res, next) {
try {
const content = await readMyFile(filePath); // the content is undefined here
res.writeHead(200, { "Content-Type": "application/json" });
res.write(JSON.stringify(content));
} catch (err) {
console.log("Error occurred.");
res.status(500).send("Error");
} finally {
res.end();
}
};
Thank you for your time!

fs.readFile uses a callback and does not return a promise which means it can't be used properly in an async function. If you want to use an async function I suggest returning a promise.
const readFile = async (filePath) => {
return new Promise((resolve, reject) => {
if (!exists(filePath)) {
reject(Error("File not found"));
}
fs.readFile(filePath, (err, data) => {
if (err) {
reject(err);
}
resolve(data)
});
})
}

Why is returning value from callback of xml-to-json getting undefined value?

const xml2json = require("xml-to-json");
const convertXMITOJSON = () => {
xml2json(
{
input: "./sequence_diagram.xmi",
output: "./test.json",
},
function (err, result) {
if (err) console.error(err);
else {
return result;
}
}
);
};
let result = convertXMITOJSON();
console.log(result); // undefined
I want to use the value of result outside of this function. But when I return the value of result, it's getting undefined. Why is in this code the value of result outside the function undefined?

Promises are the easiest way to deal with it
const xml2json = require("xml-to-json");
const convertXMITOJSON = () => {
return new Promoise((resolve, reject) => {
xml2json({
input: "./sequence_diagram.xmi",
output: "./test.json",
},
function(err, result) {
if (err) {
console.error(err);
reject(err)
} else {
resolve(result);
}
}
);
})
};
convertXMITOJSON().then(result => {
console.log(result);
}
The code could be changed with async and await

Modular Implementation of MongoDB using Promises and Async/Await

I would like to make an Ajax request to my MongoDB server and use the data along with other async tasks using a standalone function so that I can modularize my code as much as possible. I am not very experienced with async programming so I might be doing some basic mistake. In my code, I used another function (doubleAfter2Seconds) returning a promise, which works fine. The result variable from await getMongoData("mydb", url) is outputted as undefined instead of the actual data.
var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://{MyServer}:27017/";
function getMongoData(dboArg, urlArg) {
var myPromise = () => {
return new Promise(resolve => {
MongoClient.connect(urlArg, { useNewUrlParser: true }, function(err, db) {
if (err) throw err;
var dbo = db.db(dboArg);
dbo.collection(myCollection).find({}).toArray(function(err, result) {
if (err) throw err;
console.log(result);
db.close();
resolve(result[0]);
});
})
})
}
}
function doubleAfter2Seconds(x) {
return new Promise(resolve => {
console.log("v");
setTimeout(() => {
resolve(x * 2);
}, 1000);
});
}
async function addAsync(x) {
// This works
const a = await doubleAfter2Seconds(10);
console.log(a);
// This doesn't work
result = await getMongoData("mydb", url);
console.log(result);
return x;
}
addAsync(10).then((sum) => {
console.log(sum);
});

Here is the corrected getMongoData function based on the comments
function getMongoData(dboArg, urlArg) {
return new Promise(resolve => {
MongoClient.connect(urlArg, { useNewUrlParser: true }, function(err, db) {
if (err) throw err;
var dbo = db.db(dboArg);
dbo.collection(myCollection).find({}).toArray(function(err, data) {
err
? reject(err)
: resolve(data[0]);
});
})
})
}

How to delay all the calls to any method of a javascript object until the object is initialized?

Suppose this simple piece of code:
/*jslint node: true */
"use strict";
function MyClass(db) {
var self = this;
this._initError = new Error("MyClass not initialized");
db.loadDataAsyncronously(function(err, data) {
if (err) {
self._initError =err;
} else {
self._initError = null;
self.data = data;
}
});
}
MyClass.prototype.getA = function(cb) {
if (this._initError) {
return cb(this._initError);
}
return cb(null, this.data.a);
};
MyClass.prototype.getB = function(cb) {
if (this._initError) {
return cb(this._initError);
}
return cb(null, this.data.b);
};
var db = {
loadDataAsyncronously: function(cb) {
// Emulate the load process from disk.
// Data will be available later
setTimeout(function() {
cb(null, {
a:"Hello",
b:"World"
});
},1000);
}
};
var obj = new MyClass(db);
obj.getA(function (err, a) {
if (err) {
console.log(err);
} else {
console.log("a: " + a);
}
});
obj.getB(function (err, b) {
if (err) {
console.log(err);
} else {
console.log("a: " + b);
}
});
This gives an error because obj is not initialized when getA and getB methods are called. I would like that any method called before the object is initialized, be delayed automatically until the class finish its initialization.
One way to solve it is this way:
/*jslint node: true */
"use strict";
function MyClass(db) {
var self = this;
self._pendingAfterInitCalls = [];
db.loadDataAsyncronously(function(err, data) {
if (!err) {
self.data = data;
}
self._finishInitialization(err);
});
}
MyClass.prototype.getA = function(cb) {
this._waitUntiliInitialized(function(err) {
if (err) {
return cb(err);
}
return cb(null, this.data.a);
});
};
MyClass.prototype.getB = function(cb) {
this._waitUntiliInitialized(function(err) {
if (err) {
return cb(err);
}
return cb(null, this.data.b);
});
};
MyClass.prototype._finishInitialization = function(err) {
this._initialized=true;
if (err) {
this._initError = err;
}
this._pendingAfterInitCalls.forEach(function(call) {
call(err);
});
delete this._pendingAfterInitCalls;
};
MyClass.prototype._waitUntiliInitialized = function(cb) {
var bindedCall = cb.bind(this);
if (this._initialized) {
return bindedCall(this._initError);
}
this._pendingAfterInitCalls.push(bindedCall);
};
var db = {
loadDataAsyncronously: function(cb) {
// Emulate the load process from disk.
// Data will be available later
setTimeout(function() {
cb(null, {
a:"Hello",
b:"World"
});
},1000);
}
};
var obj = new MyClass(db);
obj.getA(function (err, a) {
if (err) {
console.log(err);
} else {
console.log("a: " + a);
}
});
obj.getB(function (err, b) {
if (err) {
console.log(err);
} else {
console.log("a: " + b);
}
});
But it seems to me a lot of overhead to be written for each class following this pattern.
Is there a more elegant way to handle this functionality?
Does exist any library to simplify this functionality?

Preparing this question, came to my head what probable would be a good answer. The idea is to use the concept of a factory function. The code above would be rewritten this way.
/*jslint node: true */
"use strict";
function createMyClass(db, cb) {
var obj = new MyClass();
obj._init(db, function(err) {
if (err) return cb(err);
cb(null, obj);
});
}
function MyClass() {
}
MyClass.prototype._init = function(db, cb) {
var self=this;
db.loadDataAsyncronously(function(err, data) {
if (err) return cb(err);
self.data = data;
cb();
});
};
MyClass.prototype.getA = function(cb) {
if (this._initError) return cb(this._initError);
cb(null, this.data.a);
};
MyClass.prototype.getB = function(cb) {
if (this._initError) return cb(this._initError);
cb(null, this.data.b);
};
var db = {
loadDataAsyncronously: function(cb) {
// Emulate the load process from disk.
// Data will be available later
setTimeout(function() {
cb(null, {
a:"Hello",
b:"World"
});
},1000);
}
};
var obj;
createMyClass(db,function(err, aObj) {
if (err) {
console.log(err);
return;
}
obj=aObj;
obj.getA(function (err, a) {
if (err) {
console.log(err);
} else {
console.log("a: " + a);
}
});
obj.getB(function (err, b) {
if (err) {
console.log(err);
} else {
console.log("a: " + b);
}
});
});
I share this Q/A because I thing it can be interesting to some body else. If you thing there exist better solutions, libraries to handle this situations, or any other idea, I will appreciate it.

The usual strategy here is to NOT use any async operation from a constructor. If an object needs an async operation in order to initialize it, then you use one of two options:
The async portion of the initialization is done in an .init(cb) method that must be called by the creator of the object.
You use a factory function that takes a callback that is called when the async portion of the operation has completed (like your proposed answer).
If you create a lot of these, perhaps the factory function makes sense because it saves you a little repeated code. If you don't create a lot of them, I prefer the first option because I think it makes it a lot clearer in the code exactly what is happen (you create an object and then you initialize it asynchronously and then the code continues only when the async operation has completed).
For the first option, it could look like this:
function MyClass(...) {
// initialize instance variables here
}
MyClass.prototype.init = function(db, callback) {
var self = this;
db.loadDataAsyncronously(function(err, data) {
if (err) {
self._initError = err;
} else {
self._initError = null;
self.data = data;
}
callback(err);
});
}
// usage
var obj = new MyClass(...);
obj.init(db, function(err) {
if (!err) {
// continue with rest of the code that uses this object
// in here
} else {
// deal with initialization error here
}
});
Personally, I would probably use a design using promises so the calling code could look like this:
var obj = new MyClass(...);
obj.init(db).then(function() {
// object successfully initialized, use it here
}, function(err) {
// deal with initialization error here
});

You can use a Promise internally in MyClass and have all functions wait for the promise to complete before proceeding.
http://api.jquery.com/promise/

You could inject the precondition into the functions:
function injectInitWait(fn) {
return function() {
var args = Array.prototype.slice.call(arguments, 0);
this._waitUntiliInitialized(function(err) {
if (err) return args[args.length - 1](err);
fn.apply(this, args);
});
}
}
injectInitWait(MyClass.prototype.getA);
injectInitWait(MyClass.prototype.getB);
That being said, your factory approach would be superior.

Retry an async function if the error is retryable

How can I change my logic to retry if the err.retryable = true in the following code:
async.each(queues, function (queue, callback) {
sqs.getQueueUrl({'QueueName': queue.queue}, function (err, qurl) {
if (err) {
if (err.retryable) {
// How to retry sqs.getQueueUrl({'QueueName': queue.queue}...?
} else {
console.error(err, err.stack);
callback(err);
}
}else{
//Do lots of things here
}
})
}, function (err) {
//...
})

In addition to the advice by dfsq to name your callback and use it an asynchronously recursive manner, see also async.retry from the async module by Caolan McMahon. Example:
async.retry(3, apiMethod, function(err, result) {
// do something with the result
});
More complex example:
async.auto(
{
users: api.getUsers.bind(api),
payments: async.retry(3, api.getPayments.bind(api))
}, function(err, results) {
// do something with the results
}
);
More details in the docs.
UPDATE
A better solution for your use case:
I wrote a utility function that you can use to make your original method support any number of retries (with err.retryable support).
You can use it this way:
var retryingFunction = withRetries(sqs, sqs.getQueueUrl);
(Note that you need to provide both sqs and sqs.getQueueUrl)
And now you can use the retryingFunction just like you would use sqs.getQueueUrl but with a number of retries as the first arguments. The retries will only be done when err.retryable is true.
So now, instead of:
sqs.getQueueUrl({'QueueName': queue.queue}, function (err, qurl) {
// ...
});
you can use:
retryingFunction(3, {'QueueName': queue.queue}, function (err, qurl) {
// ...
});
where 3 is the number of retries.
And this is the function that I wrote to make the above possible:
function withRetries(obj, method) {
if (!method) {
method = obj;
obj = null;
}
if (typeof method != "function") {
throw "Bad arguments to function withRetries";
}
var retFunc = function() {
var args = Array.prototype.slice.call(arguments);
var retries = args.shift();
var callback = args.pop();
if (typeof retries != "number" || typeof callback != "function") {
throw "Bad arguments to function returned by withRetries";
}
var retryCallback = function (err, result) {
if (err && err.retryable && retries > 0) {
retries--;
method.apply(obj, args);
} else {
callback(err, result);
}
};
args.push(retryCallback);
method.apply(obj, args);
};
return retFunc;
}
See this LIVE DEMO to play with it and see how it works.
It works fine in the demo, I hope it will also work for your code.

You can give queue callback a name and provide it in retry request again. Try this:
async.each(queues, function (queue, callback) {
sqs.getQueueUrl({'QueueName': queue.queue}, function queueCallback(err, qurl) {
if (err) {
if (err.retryable) {
sqs.getQueueUrl({'QueueName': queue.queue}, queueCallback);
} else {
console.error(err, err.stack);
callback(err);
}
} else {
//Do lots of things here
}
});
}, function (err) {
//...
});