Using bluebird promises, I am trying to check if a certain version of a file exists (always only ONE of those files exists but i don't know which one). To speed things up, I am using the any() collection to get all checking out concurrently:
Promise.any([
fs.existsAsync('./foo/image1.jpg'),
fs.existsAsync('./foo/image2.gif'),
fs.existsAsync('./foo/image3.png')
]).then(function(result) {
console.log(result); //remains undefined
});
I am always getting an undefined result. According to (How do I return the response from an asynchronous call?) this is normal behaviour of a resolved promise. Still, I need to know, which of my three promises resolved or maybe I just can't use any() for that purpose?
The callback for fs.exists() does not follow the expected calling convention of callback(err, value) so it doesn't work with generic promisify. It uses just callback(value).
You could either use fs.statAsync() instead or you could create your own promisification for fs.exists() that would work properly.
Of course, fs.exists() is deprecated anyway for race condition reasons so you should perhaps rethink your tactic here anyway.
Here's a properly promisified fs.existsAsync():
fs.existsAsync = function(path) {
return new Promise(function(resolve) {
fs.exists(path, resolve);
});
}
You would assign this after you've done the general promisification of the fs module to replace the wrong fs.existsAsync that Bluebird did automatically.
Related
I have a Dialog object that will show, well, dialogs. There are many entry points to show dialogs, e.g. yesNo(), message(), confirm(), etc. However, all these methods basically call the same other method, called showSimpleDialog(title, message, buttons).
I'd like all these methods (showSimpleDialog too) to return a promise, but there's a snag:
yesNo() {
return new Promise((resolve, reject) => {
axios
.get(......)
.then(this.showSimpleDialog(...));
}
}
As you can see, I am prevented in the above example from either returning the promise that showSimpleDialog would make or by passing the instanced Promise to showSimpleDialog.
The former is impossible because we're already in a different Promise by the time we have access to it. The latter because the Promise object itself is not yet available within the constructor. Well, technically, in this particular case it is (exactly because we're already in a different Promise), but some of my entry functions are synchronous, some asynchronous and I simply can't use the same code pattern to achieve the same effect in both cases.
I investigated the thing and I found this, but the author suggests the approach is flawed and archaic to begin with.
So, what would be the correct approach to return a functioning Promise from ALL entry points while the entry points would still be free to reusing each other's Promises?
If I understand correctly, this.showSimpleDialog(...) also returns a Promise, right?
If you want yesNo() to return the Promise retunred by this.showSimpleDialog(...)
yesNo() {
return axios
.get(......)
.then(()=>{
return this.showSimpleDialog(...);
});
}
That being said, consider using async/await, especially when dealing with multiple sequential promises, if possible.
Your code is calling this.showSimpleDialog immediately (synchronously) without waiting for any promise to resolve (the axios one). This is because the code doesn't pass a function to the then method, but instead executes this.showSimpleDialog. This execution returns a promise (presumably), but then expects a function as argument, not a promise.
So you need to make sure to pass a callback to then, and let that callback return a promise. This way promises will be chained:
.then(() => this.showSimpleDialog(...));
It is also important to make that callback an arrow function, since you'll be referencing this, which is intended to be the this on which yesNo is called.
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.
I'm coming from a PHP background and I'm trying to learn NodeJS.
I know that everything in Node is async but I've found that i've been using the async / await combo quite a lot in my code and I wanted to make sure I wasn't doing something stupid.
The reason for it is that I have a lot of situations where I need the result of something before continuing (for example for a small ajax request). This small ajax request has no other purpose other than to do the set of things that I want it to and in the order that I specify so even if I do things the "async way" and write it using a callback i'm still having to wait for things to finish in the right order.
Right now whenever I find myself in this situation I just use await to wait for the result:
ie:
var result = await this.doSomething(data);
Opposed to using a callback
this.doSomething(data, function(callback) {
// code
callback();
});
To me the first example looks cleaner than the second one which is why I've been opting for that. But I'm worried that I might be missing something fundamental here. But in a situation where there is nothing else to process below the async call and the only way for things to progress is for it to follow a syncronous style, is there anything wrong with using the first style over the second?
But I'm worried that I might be missing something fundamental here.
Nope, you're not, that's exactly what you want to do, assuming this.doSomething(data) is asynchronous (and if it's an ajax call, one hopes it is async) and that it returns a promise (which all functions defined with the async keyword do implicitly). (If it's not asynchronous, you don't need the await, although it's allowed.)
You're probably being a bit confused (understandably) by the fact that things are going through a transition. Until recently, the overwhelming convention in Node APIs (the built-in ones and ones provided by third-party modules) was to use the "Node callback" pattern, which is that a function that will do asynchronous work expects its last argument to be a callback, which it will call with a first argument indicating success/failure (null = success, anything else is an error object) with subsequent arguments providing the result. (Your second example assumes doSomething is one of these, instead of being a function that returns a promise.)
Example: fs.readFile, which you use like this:
fs.readFile("/some/file", "utf-8", function(err, data) {
if (err) {
// ...handle the fact an error occurred..
return;
}
// ...use the data...
});
This style quickly leads to callback hell, though, which is one of the reasons promises (aka "futures") were invented.
But a lot of Node APIs still use the old pattern.
If you need to use "Node callback"-pattern functions, you can use util.promisify to create promise-enabled versions of them. For instance, say you need to use fs.readFile, which uses the Node callback pattern. You can get a promise version like this:
const readFilePromise = util.promisify(fs.readFile);
...and then use it with async/await syntax (or use the promise directly via then):
const data = await readFilePromise("/some/file", "utf-8");
There's also an npm module called promisify that can provide a promise-ified version of an entire API. (There's probably more than one.)
Just because promises and async/await replace the old Node callback style in most cases doesn't mean callbacks don't still have a place: A Promise can only be settled once. They're for one-off things. So callbacks still have a place, such as with the on method of EventEmitters, like a readable stream:
fs.createReadStream("/some/file", "utf-8")
.on("data", chunk => {
// ...do something with the chunk of data...
})
.on("end", () => {
// ...do something with the fact the end of the stream was reached...
});
Since data will fire multiple times, it makes sense to use a callback for it; a promise wouldn't apply.
Also note that you can only use await in an async function. Consequently, you may find yourself getting into the habit of a "main" module structure that looks something like this:
// ...Setup (`require` calls, `import` once it's supported, etc.)...
(async () => {
// Code that can use `await `here...
})().catch(err => {
// Handle the fact an error/promise rejection occurred in the top level of your code
});
You can leave the catch off if you want your script to terminate on an unhandled error/rejection. (Node doesn't do that yet, but it will once unhandled rejection detection matures.)
Alternately, if you want to use a promise-enabled function in a non-async function, just use then and catch:
this.doSomething()
.then(result => {
// Use result
})
.catch(err => {
// Handle error
});
Note: async/await is directly supported in Node 7.x and above. Be sure your target production environment supports Node 7.x or above if your'e going to use async/await. If not, you could transpile your code using something like Babel and then use the transpiled result on the older version of Node.
I was reading How to use MongoDB with promises in Node.js? when I saw that in the first answer they say that if I pass no callback, mongo driver will return me a promise. It worked for 'connect' but it didn't work when I tried:
db.collection('myCollection').find().then((docs)=>{console.log(docs)})
I got:
MongoDB error: TypeError: db.collection(...).find(...).then is not a function
I tried to read the documentation for find() at http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#find and I can see some examples there that does things like this:
collection.find({}).explain().then(function(docs) {
test.ok(docs != null);
db.close();
});
this is a Promise for find but it has this explain() thing. What is it? Also, why there is no mention of promises in this API? There's also another line that does:
collection.insertMany([{a:1}, {a:2}, {a:3}], {w:1}).then(function(result) {
which is also a promise.
So, how to use promises with find()?
Also, why this documentation won't tell the return values of anything? Neither the arguments I can use...
What you are looking for is toArray(), which works like you want (callback inside, or promise returned if not callback)
db.collection('...').find().toArray()
.then(function(docs) {
// =)
});
This is because db.collection('..').find() returns a cursor, not a promise.
This behavior is intended because "find as a whole" is not the only pattern that can be used (ie. streams are allowed).
I'm aware of the power of promises, however I have several old functions that are synchronous:
function getSomething() {
return someExternalLibrary.functionReturnsAValue()
}
console.log(getSomething()); // eg prints 'foo'
Unfortunately, when someExternalLibrary updated, it has removed functionReturnsAValue() and has lumped me with functionReturnsAPromise():
function getSomething() {
return someExternalLibrary.functionReturnsAPromise()
}
console.log(getSomething()); // now prints '[object]'
This of course, breaks absolutely everything written that depends on what used to be a simple value.
Obviously, I'd prefer two things:
ask the original library to keep a synchronous return value. (Not going to happen -- b/c they have refused)
A way to actually wait for a value
I have read numerous articles on why promises are great, ad nauseam, but the simple fact is: If I embrace promises, all I really do is shuffle promises onto some other part of the code, which then must deal with the promise of a value...
Is there a way (in nodejs) to actually wait for a promise to get itself together?
The best I can find is to use coroutines and yield, but really, it's still passing the buck. To be clear, I want the function getSomething to continue to return a value. Is there a way to do it?
Clearly, I fear I've misunderstood something about Promises...
The app is for non-browser implementations and runs purely from the command line. I've been trying to understand how bluebird's reflect() might help, to no avail.
(Yes, I'm aware this question has been asked many times in various formats, but I can't find a suitable answer to the core issue. If anything, I'm looking for the opposite of this question. The closest related (but unhelpful) question I can find is: Managing promise dependencies.)
There's the concept of generator functions. These are a special kind of function in both syntax (asterisk notation) and semantics. Unlike regular functions, generator functions return something that's also new to ECMAScript: iterators. Iterators happen to be objects made specifically to be iterated on, e.g. with the all new for...of loop. They can be also iterated on manually by calling their 'next' method. Each such call produces an object containing two properties: 'value' (iterator's current value) and 'done' (a boolean indicating whether we reached the last value of the iterable). However, the best thing about generator functions is their ability to suspend their execution each time a keyword 'yield' is encountered. Let's have a glimpse of how it all works together:
'use strict';
let asyncTask = () =>
new Promise((resolve, reject) => {
if (Math.random() > 0.5) {
resolve(1);
} else {
reject(new Error('Something went wrong'));
}
});
let makeMeLookSync = fn => {
let iterator = fn();
let loop = result => {
!result.done && result.value.then(
res => loop(iterator.next(res)),
err => loop(iterator.throw(err))
);
};
loop(iterator.next());
};
makeMeLookSync(function* () {
try {
let result = yield asyncTask();
console.log(result);
} catch (err) {
console.log(err.message);
}
});
The short answer
I am told repeatedly: You can't undo functions that have been promisified.
Edit: An upcoming solution
It appears that the ES2017 (although still draft), goes a long way in making promisified code easier to work with:
https://ponyfoo.com/articles/understanding-javascript-async-await
It seems that there is also a node library ready for this support too: https://github.com/normalize/mz.
Using this methodology, having apis converted to Promises won't be so bad (although it still appears that promises still poison the rest of the codebase):
const fs = require('mz/fs')
async function doSomething () {
if (await fs.exists(__filename)) // do something
}
The rest of this answer is just a general commentary on the problem.
Why we need a solution
Let's start with a sample piece of traditional synchronous code, in 3 flavours from more 'older-fashioned' to 'newer':
This is the traditional javascript way, requiring exception based programming to handle unexpected errors:
function getSomething() {
if (someproblem) throw new Error('There is a problem');
return 'foo';
}
However, adding try/ catch statements becomes very laborious and tedious, very quickly.
With the advent of node.js, callbacks were made popular, which nicely circumvented the issue, since each caller was explicitly forced to deal with error conditions in the same callback. This meant less errors in the caller's code:
function getSomething(callback) {
if (callback) {
if (someproblem)
callback(new Error('There is a problem'), null);
else
callback(null, 'foo');
}
return 'foo';
}
Then, the after some teething issues, node.js quickly proved itself for server-side communications, and people were amazed at the speed that asynchronous solutions provided. Node application frameworks like Express and Meteor grew, which focused on this.
Unfortunately, using the same callback scheme quickly became troublesome and the developers dealing in asynchronous code started using Promises in an effort to linearize the code, to make it readable, like the traditional (try/catch) code was.
The problem is that it got evangenlized too much. Everyone started thinking that Promises are the way to go. Personally, I call it a poison on a codebase. Once you have anything that uses Promises, your whole codebase must become asynchronous. This is not always a sensible nor a practical solution, IMHO.
The worst of all side effects is that the above function, even though it is completely synchronous, can be written in Promises too:
var bluebird = require('bluebird');
function getSomething() {
// IMHO, this is ridiculous code, but is increasingly popular.
if (someproblem) return Promise.reject(new Error('There is a problem'));
return Promise.resolve('foo');
}
For those who doubt this is a problem, perhaps should look at the SO question: How do I convert an existing callback API to promises?. Pay particular attention to #3, Node-style callback.
So, for anyone who cares, I would like to suggest that there needs to be a 'pill' for Promises. I urge that we need more than promises: we need results, and sometimes in a timely manner.
Take a look at the default node.js api. It does not use Promises. It also provides both synchronous and asynchronous calls to appropriate parts of the api (eg File System).
For those of you who feel tempted to downvote this answer: that is your prerogative, but there are clear issues on when Promises are not the answer, and I feel strongly that there are cases when we need to be able to re-synchronize decoupled code.
I also apologize for this 'blog-post' styled answer.