When to use promise.all()?

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.

Answers:

Answer

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, 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.readFileAsync(...), 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():

  1. 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().
  2. When you don't need to coordinate among multiple async operations.
  3. 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.settle() instead.
  4. If your async operations do not all return promises, Promise.all() cannot track an async operation that is not managed through a promise.
Answer

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.

Answer

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.

Answer

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()

Answer

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.

Answer

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

  1. Syntax -Promise.all(iterable);
Answer

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.

Answer

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) => {
  // ?? ?? ??? 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:

  1. Create for each URL corresponding to PROMIS.
  2. 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') // (??? ????? ????????)
]).then(
    result => alert("?? ?????????"),
    error => alert("??????: " + error.message) // ??????: 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/

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