This is more of a conceptual question. I understand the Promise design pattern, but couldn't find a reliable source to answer my question about promise.all():
What is(are) the correct scenario(s) to use promise.all()
OR
Are there any best practices to use promise.all()? Should it be ideally used only if all of the promise objects are of the same or similar types?
The only one I could think of is:
Use promise.all() if you want to resolve the promise only if all of the promise objects resolve and reject if even one rejects.
I'm not sure anyone has really given the most general purpose explanation for when to use Promise.all() (and when not to use it):
What is(are) the correct scenario(s) to use promise.all()
Promise.all() is useful anytime you have more than one promise and your code wants to know when all the operations that those promises represent have finished successfully. It does not matter what the individual async operations are. If they are async, are represented by promises and your code wants to know when they have all completed successfully, then Promise.all() is built to do exactly that.
For example, suppose you need to gather information from three separate remote API calls and when you have the results from all three API calls, you then need to run some further code using all three results. That situation would be perfect for Promise.all(). You could so something like this:
Promise.all([apiRequest(...), apiRequest(...), apiRequest(...)]).then(function(results) {
// API results in the results array here
// processing can continue using the results of all three API requests
}, function(err) {
// an error occurred, process the error here
});
Promise.all() is probably most commonly used with similar types of requests (as in the above example), but there is no reason that it needs to be. If you had a different case where you needed to make a remote API request, read a local file and read a local temperature probe and then when you had data from all three async operations, you wanted to then do some processing with the data from all three, you would again use Promise.all():
Promise.all([apiRequest(...), fs.promises.readFile(...), readTemperature(...)]).then(function(results) {
// all results in the results array here
// processing can continue using the results of all three async operations
}, function(err) {
// an error occurred, process the error here
});
On the flip side, if you don't need to coordinate among them and can just handle each async operation individually, then you don't need Promise.all(). You can just fire each of your separate async operations with their own .then() handlers and no coordination between them is needed.
In addition Promise.all() has what is called a "fast fail" implementation. It returns a master promise that will reject as soon as the first promise you passed it rejects or it will resolve when all the promises have resolved. So, to use Promise.all() that type of implementation needs to work for your situation. There are other situations where you want to run multiple async operations and you need all the results, even if some of them failed. Promise.all() will not do that for you directly. Instead, you would likely use something like Promise.settle() for that situation. You can see an implementation of .settle() here which gives you access to all the results, even if some failed. This is particularly useful when you expect that some operations might fail and you have a useful task to pursue with the results from whatever operations succeeded or you want to examine the failure reasons for all the operations that failed to make decisions based on that.
Are there any best practices to use promise.all()? Should it be
ideally used only if all of the promise objects are of the same or
similar types?
As explained above, it does not matter what the individual async operations are or if they are the same type. It only matters whether your code needs to coordinate them and know when they all succeed.
It's also useful to list some situations when you would not use Promise.all():
When you only have one async operation. With only one operation, you can just use a .then() handler on the one promise and there is no reason for Promise.all().
When you don't need to coordinate among multiple async operations.
When a fast fail implementation is not appropriate. If you need all results, even if some fail, then Promise.all() will not do that by itself. You will probably want something like Promise.allSettled() instead.
If your async operations do not all return promises, Promise.all() cannot track an async operation that is not managed through a promise.
Promise.all is for waiting for several Promises to resolve in parallel (at the same time). It returns a Promise that resolves when all of the input Promises have resolved:
// p1, p2, p3 are Promises
Promise.all([p1, p2, p3])
.then(([p1Result, p2Result, p3Result]) => {
// This function is called when p1, p2 and p3 have all resolved.
// The arguments are the resolved values.
})
If any of the input Promises is rejected, the Promise returned by Promise.all is also rejected.
A common scenario is waiting for several API requests to finish so you can combine their results:
const contentPromise = requestUser();
const commentsPromise = requestComments();
const combinedContent = Promise.all([contentPromise, commentsPromise])
.then(([content, comments]) => {
// content and comments have both finished loading.
})
You can use Promise.all with Promise instance.
It's hard to answer these questions as they are the type that tend to answer themselves as one uses the available APIs of a language feature. Basically, it's fine to use Promises any way that suits your use case, so long as you avoid their anti-patterns.
What is(are) the correct scenario(s) to use promise.all()
Any situation in which an operation depends on the successful resolution of multiple promises.
Are there any best practices to use promise.all()? Should it be ideally used only if all of the promise objects are of the same or similar types?
Generally, no and no.
I use promise.all() when I have to do some requests to my API and I don't want to display something before the application loads all the data requested, so I delay the execution flow until I have all the data I need.
Example:
What I want to do I want to load the users of my app and their products (imagine that you have to do multiple requests) before displaying a table in my app with the user emails and the product names of each user.
What I do next I send the requests to my API creating the promises and using promise.all()
What I do when all the data has been loaded Once the data arrives to my app, I can execute the callback of promises.all() and then make visible the table with the users.
I hope it helps you to see in which scenario makes sense to use promises.all()
As #joews mentioned, probably one of the important features of Promise.all that should be explicitly indicated is that it makes your async code much faster.
This makes it ideal in any code that contains independent calls (that we want to return/finish before the rest of the code continues), but especially when we make frontend calls and want the user's experience to be as smooth as possible.
async function waitSecond() {
return new Promise((res, rej) => {
setTimeout(res, 1000);
});
}
function runSeries() {
console.time('series');
waitSecond().then(() => {
waitSecond().then(() => {
waitSecond().then(() => {
console.timeEnd('series');
});
});
});
}
function runParallel() {
console.time('parallel');
Promise.all([
waitSecond(),
waitSecond(),
waitSecond(),
]).then(() => {
console.timeEnd('parallel');
});
}
runSeries();
runParallel();
I tend to use promise all for something like this:
myService.getUsers()
.then(users => {
this.users = users;
var profileRequests = users.map(user => {
return myService.getProfile(user.Id); // returns a promise
});
return Promise.all(profileRequests);
})
.then(userProfilesRequest => {
// do something here with all the user profiles, like assign them back to the users.
this.users.forEach((user, index) => {
user.profile = userProfilesRequest[index];
});
});
Here, for each user we're going off and getting their profile. I don't want my promise chain to get out of hand now that i have x amount of promises to resolve.
So Promise.all() will basically aggregate all my promises back into one, and I can manage that through the next then. I can keep doing this for as long as a like, say for each profile I want to get related settings etc. etc. Each time I create tonnes more promises, I can aggregate them all back into one.
Promise.all-This method is useful for when you want to wait for more than one promise to complete or The Promise.all(iterable) method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects.
2.Just use Promise.all(files).catch(err => { })
This throws an error if ANY of the promises are rejected.
3.Use .reflect on the promises before .all if you want to wait for all
promises to reject or fulfill
Syntax -Promise.all(iterable);
Promise.all passes an array of values from all the promises in the iterable object that it was passed.
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
var isCallFailed = false;
function myEndpoint1() {
return isCallFailed ? Promise.reject("Bohoo!") :Promise.resolve({"a":"a"});
}
function myEndpoint2() {
return Promise.resolve({"b":"b"});
}
Promise.all([myEndpoint1(), myEndpoint2()])
.then(values => {
var data1 = values[0];
var data2 = values[1];
alert("SUCCESS... data1: " + JSON.stringify(data1) + "; data2: " + JSON.stringify(data2));
})
.catch(error => {
alert("ERROR... " + error);
});
you can try another case by making isCallFailed = true.
Use Promise.all only when you need to run a code according to the result of more than one asynchronous operations using promises.
For example:
You have a scenario like, You need to download 2000 mb file from server, and at the same time you are going to free the user storage to make sure it can save the downloaded file.
And you need to save only in case if the file is downloaded successfully and the storage space is created successfully.
you will do like this.
your first asynchronous operation
var p1 = new Promise(function(resolve, reject) {
// you need to download 2000mb file and return resolve if
// you successfully downloaded the file
})
and your second asynchronous operation
var p2 = new Promise(function(resolve, reject) {
// you need to clear the user storage for 2000 mb
// which can take some time
})
Now you want to save only when both of the promises resolved successfully, otherwise not.
You will use promise.all like this.
Promise.all([p1,p2]).then((result)=>{
// you will be here only if your both p1 and p2 are resolved successfully.
// you code to save the downloaded file here
})
.catch((error)=>{
// you will be here if at-least one promise in p1,p2 is rejected.
// show error to user
// take some other action
})
Promise.all can be used in a scenario when there is a routine which is validating multiplerules based on particular criteria and you have to execute them all in parallel and need to see the results of those rules at one point. Promise.all returns the results as an array which were resolved in your rule vaidator routine.
E.g.
const results = await Promise.all([validateRule1, validateRule2, validateRule3, ...]);
then results array may look like (depending upon the conditions) as for example: [true, false, false]
Now you can reject/accept the results you have based on return values. Using this way you won't have to apply multiple conditions with if-then-else.
If you are interested only Promise.all then read below Promise.all
Promise (usually they are called "Promise") - provide a convenient way to organize asynchronous code.
Promise - is a special object that contains your state. Initially, pending ( «waiting"), and then - one of: fulfilled ( «was successful") or rejected ( «done with error").
On the promise to hang callbacks can be of two types:
unFulfilled - triggered when the promise in a state of "completed
successfully."
Rejected - triggered when the promise in the "made in error."
The syntax for creating the Promise:
var promise = new Promise(function(resolve, reject) {
// This function will be called automatically
// It is possible to make any asynchronous operations,
// And when they will end - you need to call one of:
// resolve(result) on success
// reject(error) on error
})
Universal method for hanging handlers:
promise.then(onFulfilled, onRejected)
onFulfilled - a function that will be called with the result with
resolve.
onRejected - a function that will be called when an error reject.
With its help you can assign both the handler once, and only one:
// onFulfilled It works on success
promise.then(onFulfilled)
// onRejected It works on error
promise.then(null, onRejected)
Synchronous throw - the same that reject
'use strict';
let p = new Promise((resolve, reject) => {
// то же что reject(new Error("o_O"))
throw new Error("o_O");
});
p.catch(alert); // Error: o_O
Promisification
Promisification - When taking asynchronous functionality and make it a wrapper for returning PROMIS.
After Promisification functional use often becomes much more convenient.
As an example, make a wrapper for using XMLHttpRequest requests
httpGet function (url) will return PROMIS, which upon successful data loading with the url will go into fulfilled with these data, and in case of error - in rejected with an error information:
function httpGet(url) {
return new Promise(function(resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.onload = function() {
if (this.status == 200) {
resolve(this.response);
} else {
var error = new Error(this.statusText);
error.code = this.status;
reject(error);
}
};
xhr.onerror = function() {
reject(new Error("Network Error"));
};
xhr.send();
});
}
As you can see, inside the function XMLHttpRequest object is created and sent as usual, when onload / onerror are called, respectively, resolve (at the status 200) or reject.
Using:
httpGet("/article/promise/user.json")
.then(
response => alert(`Fulfilled: ${response}`),
error => alert(`Rejected: ${error}`)
);
Parallel execution
What if we want to implement multiple asynchronous processes simultaneously and to process their results?
The Promise class has the following static methods.
Promise.all(iterable)
Call Promise.all (iterable) receives an array (or other iterable object) and returns PROMIS PROMIS, which waits until all transferred PROMIS completed, and changes to the state "done" with an array of results.
For example:
Promise.all([
httpGet('/article/promise/user.json'),
httpGet('/article/promise/guest.json')
]).then(results => {
alert(results);
});
Let's say we have an array of URL.
let urls = [
'/article/promise/user.json',
'/article/promise/guest.json'
];
To download them in parallel, you need to:
Create for each URL corresponding to PROMIS.
Wrap an array of PROMIS in Promise.all.
We obtain this:
'use strict';
let urls = [
'/article/promise/user.json',
'/article/promise/guest.json'
];
Promise.all( urls.map(httpGet) )
.then(results => {
alert(results);
});
Note that if any of Promise ended with an error, the result will
Promise.all this error.
At the same time the rest of PROMIS ignored.
For example:
Promise.all([
httpGet('/article/promise/user.json'),
httpGet('/article/promise/guest.json'),
httpGet('/article/promise/no-such-page.json') // (нет такой страницы)
]).then(
result => alert("не сработает"),
error => alert("Ошибка: " + error.message) // Ошибка: Not Found
)
In total:
Promise - is a special object that stores its state, the current
result (if any), and callbacks.
When you create a new Promise ((resolve, reject) => ...) function
argument starts automatically, which should call resolve (result) on
success, and reject (error) - error.
Argument resolve / reject (only the first, and the rest are ignored)
is passed to handlers on this Promise.
Handlers are appointed by calling .then / catch.
To transfer the results from one processor to another using Channing.
https://www.promisejs.org/patterns/
Related
I've been developing JavaScript for a few years and I don't understand the fuss about promises at all.
It seems like all I do is change:
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Which I could use a library like async for anyway, with something like:
api().then(function(result){
api2().then(function(result2){
api3().then(function(result3){
// do work
});
});
});
Which is more code and less readable. I didn't gain anything here, it's not suddenly magically 'flat' either. Not to mention having to convert things to promises.
So, what's the big fuss about promises here?
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
});
Certainly, not much less code, but much more readable.
But this is not the end. Let's discover the true benefits: What if you wanted to check for any error in any of the steps? It would be hell to do it with callbacks, but with promises, is a piece of cake:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
});
Pretty much the same as a try { ... } catch block.
Even better:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
}).then(function() {
//do something whether there was an error or not
//like hiding an spinner if you were performing an AJAX request.
});
And even better: What if those 3 calls to api, api2, api3 could run simultaneously (e.g. if they were AJAX calls) but you needed to wait for the three? Without promises, you should have to create some sort of counter. With promises, using the ES6 notation, is another piece of cake and pretty neat:
Promise.all([api(), api2(), api3()]).then(function(result) {
//do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
//handle the error. At least one of the promises rejected.
});
Hope you see Promises in a new light now.
Yes, Promises are asynchronous callbacks. They can't do anything that callbacks can't do, and you face the same problems with asynchrony as with plain callbacks.
However, Promises are more than just callbacks. They are a very mighty abstraction, allow cleaner and better, functional code with less error-prone boilerplate.
So what's the main idea?
Promises are objects representing the result of a single (asynchronous) computation. They resolve to that result only once. There's a few things what this means:
Promises implement an observer pattern:
You don't need to know the callbacks that will use the value before the task completes.
Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object
The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot.
You can add multiple callbacks easily
Promises are chainable (monadic, if you want):
If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks.
If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.
Sounds complicated? Time for a code example.
var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
var p2 = api2(); // returning a promise
return p2; // The result of p2 …
}); // … becomes the result of p3
// So it does not make a difference whether you write
api1().then(function(api1Result) {
return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
return api2();
}).then(console.log)
Flattening does not come magically, but you can easily do it. For your heavily nested example, the (near) equivalent would be
api1().then(api2).then(api3).then(/* do-work-callback */);
If seeing the code of these methods helps understanding, here's a most basic promise lib in a few lines.
What's the big fuss about promises?
The Promise abstraction allows much better composability of functions. For example, next to then for chaining, the all function creates a promise for the combined result of multiple parallel-waiting promises.
Last but not least Promises come with integrated error handling. The result of the computation might be that either the promise is fulfilled with a value, or it is rejected with a reason. All the composition functions handle this automatically and propagate errors in promise chains, so that you don't need to care about it explicitly everywhere - in contrast to a plain-callback implementation. In the end, you can add a dedicated error callback for all occurred exceptions.
Not to mention having to convert things to promises.
That's quite trivial actually with good promise libraries, see How do I convert an existing callback API to promises?
In addition to the already established answers, with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant. That is about to collapse into a supernova:
api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))
As oligofren pointed out, without arguments between api calls you don't need the anonymous wrapper functions at all:
api().then(api2).then(api3).then(r3 => console.log(r3))
And finally, if you want to reach a supermassive black hole level, Promises can be awaited:
async function callApis() {
let api1Result = await api();
let api2Result = await api2(api1Result);
let api3Result = await api3(api2Result);
return api3Result;
}
In addition to the awesome answers above, 2 more points may be added:
1. Semantic difference:
Promises may be already resolved upon creation. This means they guarantee conditions rather than events. If they are resolved already, the resolved function passed to it is still called.
Conversely, callbacks handle events. So, if the event you are interested in has happened before the callback has been registered, the callback is not called.
2. Inversion of control
Callbacks involve inversion of control. When you register a callback function with any API, the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run.
Refer The Javascript Event loop for an explanation.
With Promises, control resides with the calling program. The .then() method may be called at any time if we store the promise object.
In addition to the other answers, the ES2015 syntax blends seamlessly with promises, reducing even more boilerplate code:
// Sequentially:
api1()
.then(r1 => api2(r1))
.then(r2 => api3(r2))
.then(r3 => {
// Done
});
// Parallel:
Promise.all([
api1(),
api2(),
api3()
]).then(([r1, r2, r3]) => {
// Done
});
Promises are not callbacks, both are programming idioms that facilitate async programming. Using an async/await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom. A comparison of these idioms across different programming languages (including Javascript) is here: https://github.com/KjellSchubert/promise-future-task
No, Not at all.
Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished. So how it happens?
Actually, In JavaScript, functions are itself considered as objects and hence as all other objects, even functions can be sent as arguments to other functions. The most common and generic use case one can think of is setTimeout() function in JavaScript.
Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks.
The Promise receives two Callbacks in constructor function: resolve and reject. These callbacks inside promises provide us with fine-grained control over error handling and success cases. The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases.
No promises are just wrapper on callbacks
example
You can use javascript native promises with node js
my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
}
)
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums
JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected, therefore both are not fundamentally different. The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions, but it would be more readable.
Promises overview:
In JS we can wrap asynchronous operations (e.g database calls, AJAX calls) in promises. Usually we want to run some additional logic on the retrieved data. JS promises have handler functions which process the result of the asynchronous operations. The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations.
A promise always has of the 3 following states:
pending: starting state of every promise, neither fulfilled nor rejected.
fulfilled: The operation completed successfully.
rejected: The operation failed.
A pending promise can be resolved/fullfilled or rejected with a value. Then the following handler methods which take callbacks as arguments are called:
Promise.prototype.then() : When the promise is resolved the callback argument of this function will be called.
Promise.prototype.catch() : When the promise is rejected the callback argument of this function will be called.
Although the above methods skill get callback arguments they are far superior than using
only callbacks here is an example that will clarify a lot:
Example
function createProm(resolveVal, rejectVal) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (Math.random() > 0.5) {
console.log("Resolved");
resolve(resolveVal);
} else {
console.log("Rejected");
reject(rejectVal);
}
}, 1000);
});
}
createProm(1, 2)
.then((resVal) => {
console.log(resVal);
return resVal + 1;
})
.then((resVal) => {
console.log(resVal);
return resVal + 2;
})
.catch((rejectVal) => {
console.log(rejectVal);
return rejectVal + 1;
})
.then((resVal) => {
console.log(resVal);
})
.finally(() => {
console.log("Promise done");
});
The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second
If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback
If the promise is rejected the first catch method is called and the rejected value is passed in as an argument
The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function.
Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.
Promises allows programmers to write simpler and far more readable code than by using callbacks.
In a program, there are steps want to do in series.
function f() {
step_a();
step_b();
step_c();
...
}
There's usually information carried between each step.
function f() {
const a = step_a( );
const b = step_b( a );
const c = step_c( b );
...
}
Some of these steps can take a (relatively) long time, so sometimes you want to do them in parallel with other things. One way to do that is using threads. Another is asynchronous programming. (Both approaches has pros and cons, which won't be discussed here.) Here, we're talking about asynchronous programming.
The simple way to achieve the above when using asynchronous programming would be to provide a callback which is called once a step is complete.
// step_* calls the provided function with the returned value once complete.
function f() {
step_a(
function( a )
step_b(
function( b )
step_c(
...
)
},
)
},
)
}
That's quite hard to read. Promises offer a way to flatten the code.
// step_* returns a promise.
function f() {
step_a()
.then( step_b )
.then( step_c )
...
}
The object returned is called a promise because it represents the future result (i.e. promised result) of the function (which could be a value or an exception).
As much as promises help, it's still a bit complicated to use promises. This is where async and await come in. In a function declared as async, await can be used in lieu of then.
// step_* returns a promise.
async function f()
const a = await step_a( );
const b = await step_b( a );
const c = await step_c( b );
...
}
This is undeniably much much more readable than using callbacks.
I am trying to create an array of Promises, then resolve them with Promise.all(). I am using got, which returns a promise.
My code works, but I don't fully understand how. Here it is:
const got = require('got');
const url = 'myUrl';
const params = ['param1', 'param2', 'param3'];
let promiseArray = [];
for (param of params) {
promiseArray.push(got(url + param));
}
// Inspect the promises
for (promise of promiseArray) {
console.log(JSON.stringify(promise));
// Output: promise: {"_pending":true,"_canceled":false,"_promise":{}}
}
Promise.all(promiseArray).then((results) => {
// Operate on results - works just fine
}).catch((e) => {
// Error handling logic
});
What throws me off is that the Promises are marked as "pending" when I add them into the array, which means they have already started.
I would think that they should lie inactive in promiseArray, and Promise.all(promiseArray) would both start them and resolve them.
Does this mean I am starting them twice?
You're not starting them twice. Promises start running as soon as they're created - or as soon as the JS engine finds enough resources to start them. You have no control on when they actually start.
All Promise.all() does is wait for all of them to settle (resolve or reject). Promise.all() does not interfere with nor influence the order/timing of execution of the promise itself.
Promises don't run at all. They are simply a notification system for communicating when asynchronous operations are complete.
So, as soon as you ran this:
promiseArray.push(got(url + param));
Your asynchronous operation inside of got() is already started and when it finishes, it will communicate that back through the promise.
All Promise.all() does is monitor all the promises and tell you when the first one rejects or when all of them have completed successfully. It does not "control" the async operations in any way. Instead, you start the async operations and they communicate back through the promises. You control when you started the async operations and the async operations then run themselves from then on.
If you break down your code a bit into pieces, here's what happens in each piece:
let promiseArray = [];
for (param of params) {
promiseArray.push(got(url + param));
}
This calls got() a bunch of times starting whatever async operation is in that function. got() presumably returns a promise object which is then put into your promiseArray. So, at this point, the async operations are all started already and running on their own.
// Inspect the promises
for (promise of promiseArray) {
console.log(JSON.stringify(promise));
// Output: promise: {"_pending":true,"_canceled":false,"_promise":{}}
}
This loop, just looks at all the promises to see if any of them might already be resolved, though one would not expect them to be because their underlying async operations were just started in the prior loop.
Promise.all(promiseArray).then((results) => {
// Operate on results - works just fine
}).catch((e) => {
// Error handling logic
});
Then, with Promise.all(), you're just asking to monitor the array of promises so it will tell you when either there's a rejected promise or when all of them complete successfully.
Promises "start" when they are created, i.e. the function that gives you the promise, has already launched the (often asynchronous) operations that will eventually lead into an asynchronous result. For instance, if a function returns a promise for a result of an HTTP request, it has already launched that HTTP request when returning you the promise object.
No matter what you do or not do with that promise object, that function (got) has already created a callback function which it passed on to an asynchronous API, such as a HTTP Request/Response API. In that callback function (which you do not see unless you inspect the source of got) the promise will be resolved as soon as it gets called back by that API. In the HTTP request example, the API calls that particular callback with the HTTP response, and the said callback function then resolve the promise.
Given all this, it is a bit strange to think of promises as things that "start" or "run". They are merely created in a pending state. The remaining thing is a pending callback from some API that will hopefully occur and then will change the state of the promise object, triggering then callbacks.
Please note that fetching an array of urls with Promise.all has got some possible problems:
If any of the urls fail to fetch your resolve is never called (so
one fails and your resolve function is never called.
If your array is very large you will clobber the site and your network with requests, you may want to throttle the maximum open requests and or requests made in a certain timeframe.
The first problem is easily solved, you process the failed requests and add them to the results. In the resolve handler you can decide what to do with the failed requests:
const got = require('got');
const url = 'myUrl';
const params = ['param1', 'param2', 'param3'];
const Fail = function(details){this.details = details;};
Promise.all(
params.map(
param =>
got(url + param)
.then(
x=>x,//if resolved just pass along the value
reject=>new Fail([reject,url+param])
)
)
).then((results) => {
const successes = results.filter(result=>(result && result.constructor)!==Fail),
const failedItems = results.filter(result=>(result && result.constructor)===Fail);
}).catch((e) => {
// Error handling logic
});
Point 2 is a bit more complicated, throttling can be done with this helper function and would look something like this:
... other code
const max5 = throttle(5);
Promise.all(
params.map(
param =>
max5(got)(url + param)
.then(
x=>x,//if resulved just pass along the value
reject=>new Fail([reject,url+param])
)
)
)
According to MDN:
If any of the passed in promises rejects, the all Promise immediately rejects with the value of the promise that rejected, discarding all the other promises whether or not they have resolved.
The ES6 spec seems to confirm this.
My question is: Why does Promise.all discard promises if any of them reject, since I would expect it to wait for "all" promises to settle, and what exactly does "discard" mean? (It's hard to tell what "discard" means for in-flight promises vs. promises that may not have run yet.)
I ask because I frequently run into situations where I have a list of promises and want to wait for them all to settle and get all rejections that may have occurred, which Promise.all doesn't cater to. Instead, I have to use a hack like this:
const promises = []; // Array of promises
const settle = promise => promise.then(result => ({ result }), reason => ({ reason }));
Promise.all(promises.map(settle))
.then(/ * check "reason" property in each element for rejection */);
Because Promise.all guarantees they all succeeded. Simple as that.
It's the most useful building block along with Promise.race. Everything else can be built on those.
There's no settle, because it's so trivial to build like this:
Promise.all([a(), b(), c()].map(p => p.catch(e => e)))
There's no easy way to build Promise.all on top of settle, which may be why it's not the default. settle would also have had to standardize a way to distinguish success values from errors, which may be subjective and depend on the situation.
Update: There's now a Promise.allSettled that does exactly this.
The asynchronous operations associated with the promises are all run. If one of those promises rejects, then Promise.all() simply does not wait for all of them to complete, it rejects when the first promise rejects. That is just how it was designed to work. If you need different logic (like you want to wait for all of them to be done, no matter whether they fulfill or reject), then you can't use just Promise.all().
Remember, a promise is not the async operation itself. A promise is just an object that keeps track of the state of the async operation. So, when you pass an array of promises to Promise.all(), all those async operations have already been started and are all in-flight already. They won't be stopped or cancelled.
Why does Promise.all discard promises if any of them reject, since I would expect it to wait for "all" promises to settle.
It works the way it does because that's how it was designed and that is a very common use case when you don't want your code to continue if there was any sort of error. If it happens to not be your use case, then you need to use some implementation of .settle() which has the behavior you want (which you seem to already know).
What I find the more interesting question is why is there not a .settle() option in the specification and standard implementation since it is also a fairly common use case. Fortunately, as you have found, it is not a lot of code to make your own. When I don't need the actual reject reason and just want some indicator value to be placed into the array, I often use this fairly simple to use version:
// settle all promises. For rejeted promises, return a specific rejectVal that is
// distinguishable from your successful return values (often null or 0 or "" or {})
Promise.settleVal = function(rejectVal, promises) {
return Promise.all(promises.map(function(p) {
// make sure any values or foreign promises are wrapped in a promise
return Promise.resolve(p).catch(function(err) {
// instead of rejection, just return the rejectVal (often null or 0 or "" or {})
return rejectVal;
});
}));
};
// sample usage:
Promise.settleVal(null, someArrayOfPromises).then(function(results) {
results.forEach(function(r) {
// log successful ones
if (r !== null) {
console.log(r);
}
});
});
what exactly does "discard" mean?
It just means that the promises are no longer tracked by Promise.all(). The async operations they are associated with keep right on doing whatever they were going to do. And, in fact if those promises have .then() handlers on them, they will be called just as they normally would. discard does seem like an unfortunate term to use here. Nothing happens other than Promise.all() stops paying attention to them.
FYI, if I want a more robust version of .settle() that keeps track of all results and reject reasons, then I use this:
// ES6 version of settle that returns an instanceof Error for promises that rejected
Promise.settle = function(promises) {
return Promise.all(promises.map(function(p) {
// make sure any values or foreign promises are wrapped in a promise
return Promise.resolve(p).catch(function(err) {
// make sure error is wrapped in Error object so we can reliably detect which promises rejected
if (err instanceof Error) {
return err;
} else {
var errObject = new Error();
errObject.rejectErr = err;
return errObject;
}
});
}));
}
// usage
Promise.settle(someArrayOfPromises).then(function(results) {
results.forEach(function(r) {
if (r instanceof Error) {
console.log("reject reason", r.rejectErr);
} else {
// fulfilled value
console.log("fulfilled value:", r);
}
});
});
This resolves to an array of results. If a result is instanceof Error, then it was a rejected, otherwise it's a fulfilled value.
I'd argue, because rejecting a promise is like throwing an error in sync code, and an uncatched error in sync code also interrupts the execution.
Or one could argue for the assumption that requesting all these promises and composing them into a combined Array, and then waiting for all these to finish implies that you need them all to proceed with whatever you've intended to do, and if one of them fails your intended task lacks one of its dependencies, and it is logical to simply forward the reason for the fail till this reason is somehow handled/caught.
I've been developing JavaScript for a few years and I don't understand the fuss about promises at all.
It seems like all I do is change:
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Which I could use a library like async for anyway, with something like:
api().then(function(result){
api2().then(function(result2){
api3().then(function(result3){
// do work
});
});
});
Which is more code and less readable. I didn't gain anything here, it's not suddenly magically 'flat' either. Not to mention having to convert things to promises.
So, what's the big fuss about promises here?
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
});
Certainly, not much less code, but much more readable.
But this is not the end. Let's discover the true benefits: What if you wanted to check for any error in any of the steps? It would be hell to do it with callbacks, but with promises, is a piece of cake:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
});
Pretty much the same as a try { ... } catch block.
Even better:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
}).then(function() {
//do something whether there was an error or not
//like hiding an spinner if you were performing an AJAX request.
});
And even better: What if those 3 calls to api, api2, api3 could run simultaneously (e.g. if they were AJAX calls) but you needed to wait for the three? Without promises, you should have to create some sort of counter. With promises, using the ES6 notation, is another piece of cake and pretty neat:
Promise.all([api(), api2(), api3()]).then(function(result) {
//do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
//handle the error. At least one of the promises rejected.
});
Hope you see Promises in a new light now.
Yes, Promises are asynchronous callbacks. They can't do anything that callbacks can't do, and you face the same problems with asynchrony as with plain callbacks.
However, Promises are more than just callbacks. They are a very mighty abstraction, allow cleaner and better, functional code with less error-prone boilerplate.
So what's the main idea?
Promises are objects representing the result of a single (asynchronous) computation. They resolve to that result only once. There's a few things what this means:
Promises implement an observer pattern:
You don't need to know the callbacks that will use the value before the task completes.
Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object
The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot.
You can add multiple callbacks easily
Promises are chainable (monadic, if you want):
If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks.
If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.
Sounds complicated? Time for a code example.
var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
var p2 = api2(); // returning a promise
return p2; // The result of p2 …
}); // … becomes the result of p3
// So it does not make a difference whether you write
api1().then(function(api1Result) {
return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
return api2();
}).then(console.log)
Flattening does not come magically, but you can easily do it. For your heavily nested example, the (near) equivalent would be
api1().then(api2).then(api3).then(/* do-work-callback */);
If seeing the code of these methods helps understanding, here's a most basic promise lib in a few lines.
What's the big fuss about promises?
The Promise abstraction allows much better composability of functions. For example, next to then for chaining, the all function creates a promise for the combined result of multiple parallel-waiting promises.
Last but not least Promises come with integrated error handling. The result of the computation might be that either the promise is fulfilled with a value, or it is rejected with a reason. All the composition functions handle this automatically and propagate errors in promise chains, so that you don't need to care about it explicitly everywhere - in contrast to a plain-callback implementation. In the end, you can add a dedicated error callback for all occurred exceptions.
Not to mention having to convert things to promises.
That's quite trivial actually with good promise libraries, see How do I convert an existing callback API to promises?
In addition to the already established answers, with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant. That is about to collapse into a supernova:
api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))
As oligofren pointed out, without arguments between api calls you don't need the anonymous wrapper functions at all:
api().then(api2).then(api3).then(r3 => console.log(r3))
And finally, if you want to reach a supermassive black hole level, Promises can be awaited:
async function callApis() {
let api1Result = await api();
let api2Result = await api2(api1Result);
let api3Result = await api3(api2Result);
return api3Result;
}
In addition to the awesome answers above, 2 more points may be added:
1. Semantic difference:
Promises may be already resolved upon creation. This means they guarantee conditions rather than events. If they are resolved already, the resolved function passed to it is still called.
Conversely, callbacks handle events. So, if the event you are interested in has happened before the callback has been registered, the callback is not called.
2. Inversion of control
Callbacks involve inversion of control. When you register a callback function with any API, the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run.
Refer The Javascript Event loop for an explanation.
With Promises, control resides with the calling program. The .then() method may be called at any time if we store the promise object.
In addition to the other answers, the ES2015 syntax blends seamlessly with promises, reducing even more boilerplate code:
// Sequentially:
api1()
.then(r1 => api2(r1))
.then(r2 => api3(r2))
.then(r3 => {
// Done
});
// Parallel:
Promise.all([
api1(),
api2(),
api3()
]).then(([r1, r2, r3]) => {
// Done
});
Promises are not callbacks, both are programming idioms that facilitate async programming. Using an async/await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom. A comparison of these idioms across different programming languages (including Javascript) is here: https://github.com/KjellSchubert/promise-future-task
No, Not at all.
Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished. So how it happens?
Actually, In JavaScript, functions are itself considered as objects and hence as all other objects, even functions can be sent as arguments to other functions. The most common and generic use case one can think of is setTimeout() function in JavaScript.
Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks.
The Promise receives two Callbacks in constructor function: resolve and reject. These callbacks inside promises provide us with fine-grained control over error handling and success cases. The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases.
No promises are just wrapper on callbacks
example
You can use javascript native promises with node js
my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
}
)
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums
JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected, therefore both are not fundamentally different. The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions, but it would be more readable.
Promises overview:
In JS we can wrap asynchronous operations (e.g database calls, AJAX calls) in promises. Usually we want to run some additional logic on the retrieved data. JS promises have handler functions which process the result of the asynchronous operations. The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations.
A promise always has of the 3 following states:
pending: starting state of every promise, neither fulfilled nor rejected.
fulfilled: The operation completed successfully.
rejected: The operation failed.
A pending promise can be resolved/fullfilled or rejected with a value. Then the following handler methods which take callbacks as arguments are called:
Promise.prototype.then() : When the promise is resolved the callback argument of this function will be called.
Promise.prototype.catch() : When the promise is rejected the callback argument of this function will be called.
Although the above methods skill get callback arguments they are far superior than using
only callbacks here is an example that will clarify a lot:
Example
function createProm(resolveVal, rejectVal) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (Math.random() > 0.5) {
console.log("Resolved");
resolve(resolveVal);
} else {
console.log("Rejected");
reject(rejectVal);
}
}, 1000);
});
}
createProm(1, 2)
.then((resVal) => {
console.log(resVal);
return resVal + 1;
})
.then((resVal) => {
console.log(resVal);
return resVal + 2;
})
.catch((rejectVal) => {
console.log(rejectVal);
return rejectVal + 1;
})
.then((resVal) => {
console.log(resVal);
})
.finally(() => {
console.log("Promise done");
});
The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second
If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback
If the promise is rejected the first catch method is called and the rejected value is passed in as an argument
The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function.
Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.
Promises allows programmers to write simpler and far more readable code than by using callbacks.
In a program, there are steps want to do in series.
function f() {
step_a();
step_b();
step_c();
...
}
There's usually information carried between each step.
function f() {
const a = step_a( );
const b = step_b( a );
const c = step_c( b );
...
}
Some of these steps can take a (relatively) long time, so sometimes you want to do them in parallel with other things. One way to do that is using threads. Another is asynchronous programming. (Both approaches has pros and cons, which won't be discussed here.) Here, we're talking about asynchronous programming.
The simple way to achieve the above when using asynchronous programming would be to provide a callback which is called once a step is complete.
// step_* calls the provided function with the returned value once complete.
function f() {
step_a(
function( a )
step_b(
function( b )
step_c(
...
)
},
)
},
)
}
That's quite hard to read. Promises offer a way to flatten the code.
// step_* returns a promise.
function f() {
step_a()
.then( step_b )
.then( step_c )
...
}
The object returned is called a promise because it represents the future result (i.e. promised result) of the function (which could be a value or an exception).
As much as promises help, it's still a bit complicated to use promises. This is where async and await come in. In a function declared as async, await can be used in lieu of then.
// step_* returns a promise.
async function f()
const a = await step_a( );
const b = await step_b( a );
const c = await step_c( b );
...
}
This is undeniably much much more readable than using callbacks.
Question:
Is there an "easy" way to cancel ($q-/$http-)promises in AngularJS or determine the order in which promises were resolved?
Example
I have a long running calculation and i request the result via $http. Some actions or events require me to restart the calculation (and thus sending a new $http request) before the initial promise is resolved. Thus i imagine i can't use a simple implementation like
$http.post().then(function(){
//apply data to view
})
because I can't ensure that the responses come back in the order in which i did send the requests - after all i want to show the result of the latest calculation when all promises were resolved properly.
However I would like to avoid waiting for the first response until i send a new request like this:
const timeExpensiveCalculation = function(){
return $http.post().then(function(response){
if (isNewCalculationChained) {return timeExpensiveCalculation();}
else {return response.data;}
})
}
Thoughts:
When using $http i can access the config-object on the response to use some timestamps or other identifiers to manually order the incoming responses. However i was hoping I could just tell angular somehow to cancel an outdated promise and thus not run the .then() function when it gets resolved.
This does not work without manual implementation for $q-promises instead of $http though.
Maybe just rejecting the promise right away is the way to go? But in both cases it might take forever until finally a promise is resolved before the next request is generated (which leads to an empty view in the meantime).
Is there some angular API-Function that i am missing or are there robust design patterns or "tricks" with promise chaining or $q.all to handle multiple promises that return the "same" data?
I do it by generating a requestId, and in the promise's then() function I check if the response is coming from the most recent requestId.
While this approach does not actually cancel the previous promises, it does provide a quick and easy way to ensure that you are handling the most recent request's response.
Something like:
var activeRequest;
function doRequest(params){
// requestId is the id for the request being made in this function call
var requestId = angular.toJson(params); // I usually md5 hash this
// activeRequest will always be the last requestId sent out
activeRequest = requestId;
$http.get('/api/something', {data: params})
.then(function(res){
if(activeRequest == requestId){
// this is the response for last request
// activeRequest is now handled, so clear it out
activeRequest = undefined;
}
else {
// response from previous request (typically gets ignored)
}
});
}
Edit:
On a side-note, I wanted to add that this concept of tracking requestId's can also be applied to preventing duplicate requests. For example, in my Data service's load(module, id) method, I do a little process like this:
generate the requestId based on the URL + parameters.
check in requests hash-table for the requestId
if requestId is not found: generate new request and store promise in hash-table
if requestId is found: simply return the promise from the hash-table
When the request finishes, remove the requestId's entry from the hash-table.
Cancelling a promise is just making it not invoke the onFulfilled and onRejected functions at the then stage. So as #user2263572 mentioned it's always best to let go the promise not cancelled (ES6 native promises can not be cancelled anyways) and handle this condition within it's then stage (like disregarding the task if a global variable is set to 2 as shown in the following snippet) and i am sure you can find tons of other ways to do it. One example could be;
Sorry that i use v (looks like check character) for resolve and x (obvious) for reject functions.
var prom1 = new Promise((v,x) => setTimeout(v.bind(null,"You shall not read this"),2000)),
prom2,
validPromise = 1;
prom1.then(val => validPromise === 1 && console.log(val));
// oh what have i done..!?! Now i have to fire a new promise
prom2 = new Promise((v,x) => setTimeout(v.bind(null,"This is what you will see"),3000));
validPromise = 2;
prom2.then(val => validPromise === 2 && console.log(val));
I'm still trying to figure out a good way to unit test this, but you could try out this kind of strategy:
var canceller = $q.defer();
service.sendCalculationRequest = function () {
canceller.resolve();
return $http({
method: 'GET',
url: '/do-calculation',
timeout: canceller.promise
});
};
In ECMA6 promises, there is a Promise.race(promiseArray) method. This takes an array of promises as its argument, and returns a single promise. The first promise to resolve in the array will hand off its resolved value to the .then of the returned promise, while the other array promises that came in second, etc., will not be waited upon.
Example:
var httpCall1 = $http.get('/api/something', {data: params})
.then(function(val) {
return {
id: "httpCall1"
val: val
}
})
var httpCall2 = $http.get('/api/something-else', {data: params})
.then(function(val) {
return {
id: "httpCall2"
val: val
}
})
// Might want to make a reusable function out of the above two, if you use this in Production
Promise.race([httpCall1, httpCall2])
.then(function(winningPromise) {
console.log('And the winner is ' + winningPromise.id);
doSomethingWith(winningPromise.val);
});
You could either use this with a Promise polyfil, or look into the q.race that someone's developed for Angular (though I haven't tested it).