let textProcess = new Promise((resolve, reject) => {
let text
try {
// fetch text from the internet
text = "str"
resolve(text)
} catch (e) {
reject("failed to fetch!")
}
})
textProcess.then(data => {
let json
try {
json = JSON.parse(data)
} catch (e) {
console.error("failed to parse!")
// ..........i want to end the whole process here, never go ahead
}
}, e => {
// try to fetch text from local chache
}).then(json => {
// work on the json obj
}, e => {
// if failed to fetch from local too, just let it go
})
Is there a way to end a thenable chain?
Look at the example above, I want to end the whole process when parsing is failed(the line preceeded with ".........."). But actually the last then will still be invoked though.
What is the proper and elegant way to achieve my goal?
Your Promise usage involves quite a bit of sub-optimal patterns. Fixing them actually leads to what you're trying to achieve too.
textProcess.then(data => {
// 1st anti-pattern fix
// any error triggered here
// also gets caught at the end catch
return JSON.parse(data)
}).then(json => {
// work on json obj
}).catch(e => {
// 2nd anti-pattern fix
// one catch for the whole thenable chain
console.error("Failed to parse!", e)
})
This way, you properly leverage what Javascript Promise offers, and one simple .catch for what you need.
Edit - some explanations on involved Promise anti-patterns
The marked 1st anti-pattern is about unnecessary nested try..catch block within then. Within it, you can return synchronously (even undefined), another Promise (both of these are thenable), or throw an Error (which would get caught by catch). Basically you don't need to explicitly catch it but let it "flow" through.
The 2nd anti-pattern as mentioned is the fact that the second parameter -- reject handler of then is considered sub-optimal in most use cases. A Promise chain should be leveraging one catch to simplify the workflow.
However, in the rare event of the need to perform "early catch" and "resume", consider the following way, which is still a bit clearer than using two handlers for then:
textProcess.then(data => {
return parser1(data)
}).catch(e => {
console.log("First parser failed")
// for example first parser failed
return "fallback data"
}).then(data => {
// process received data as "fallback data"
}).catch(e => {
// always have a "last resort" catch at the end of the workflow
})
The short answer is no, there is no mechanism to terminate a promise chain in a then handler part-way down the chain. (A proposal for promise cancellation was made to the TC39 committee in 2016 but was subsequently withdrawn.)
Note that a "promise chain" often refers to the promise
returned by the last then, catch or finally call in a chain of single promise method calls concatenated together.
All calls to the promise methods listed above are made synchronously when code defining the chain is executed. After execution, all promises in the chain have been created and all method calls in the chain called.
Since promises only have three states (pending, fulfilled and rejected), the best you can do is to arrange for "cancellation" to be sent down the rejection channel of linked promises and ignore it as required. (There is no standard "cancelled" rejection value to use).
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/
I run into this every now and then:
return somethingThatReturnsAPromise()
.then((response) => {
soSomethingg(); // Eg; update the UI
return response;
});
Now I'm looking for something that is not expected to return anything and won't change the promise chain if I forget that:
return somethingThatReturnsAPromise()
.whatImLookingFor((response) => {
doSomething(); // Eg; update the UI
})
.then((response) => {
// and this one should still be able to access response
});
Maybe this goes against the idea of promises, but for me, it's a bit inconvenient since I can't pass arbitrary functions.
One idea is to compose a function:
const sideEffect = (callback) => {
return (response) => {
callback(response);
return response;
};
};
And I could use it as
return somethingThatReturnsAPromise()
.then(sideEffect(doSomething));
But I'd prefer something instead of then is there something like that?
Note: I'm working with Angular 1.x so I need something like for that.
I would assume that you're not really writing .then().then(), because you could collapse that into a single .then, but that your concern is really about returning the promise and having some external code add another then to the chain. In that case do this:
let p = somethingThatReturnsAPromise();
p.then(() => doSomething());
return p;
This allows the caller to attach additional thens to the original promise instead of chaining off of your .then, thereby receiving the original promise's value. This is called branching the promise chain.
Maybe this goes against the idea of promises
Slightly, promise chains are pipelines where then handlers transform things at each stage. But it's perfectly valid to want to pass through the value unchanged.
One idea is to compose a function:
Indeed the first thing that came to mind, and how I'd do it.
But I'd prefer something instead of then is there something like that?
There isn't. You could add it for your own projects (I wouldn't in a library) by adding it to Promise.prototype. Or you could give yourselve a Promise subclass and add it there.
With a Promise sublass you'd do something like:
return MyPromise.resolve(somethingThatReturnsAPromise())
.thenSide(soSomethingg); // Eg; update the UI
...where thenSide is your method that's then but passing the original value back unchanged, e.g.:
class MyPromise extends Promise {
thenSide(callback) {
this.then(callback);
return this;
}
}
or
class MyPromise extends Promise {
thenSide(callback) {
this.then(callback);
return MyPromise.resolve(this);
}
}
...depending on whether you're bothered about thenSide returning the same promise (since then always returns a new one).
As far as I know (I could well be wrong) the wrapper method for "pass-through" side-effects is an idiomatic way to do what you want.
Alternatively (if you need the same response in multiple places) you can break up the promise chain when you encounter a situation like this.
I'm doing some reading up on JS Promises to up-skill.
Here's my quandry:
Say you want to console.log('we done, bruh!') AFTER your data's come back.
so with a Promise, you might say:
let iWantToLogOut = function() {
let data = fetch('https://jsonplaceholder.typicode.com/users')
return new Promise((resolve) => {
resolve(data)
})
}
And then resolve that promise like:
iWantToLogOut().then((dataBack)
=> databack.json())
.then((json) => {
console.log('We done, bruh! Look: ', json)
})
So that's great. You get your API data back and then we log our msg out.
But isn't it just way easier to go:
let data = fetch('https://jsonplaceholder.typicode.com/users');
data ? console.log('we done, bruh!') : null;
I'm probably over-simplifying/missing something (because... well... i'm retarded) but I just want to make sure i'm really understanding Promises first before i move onto Async/Await.
But isn't it just way easier to go:
let data = fetch('https://jsonplaceholder.typicode.com/users');
data ? console.log('we done, bruh!') : null;
It would be, but it doesn't work. What fetch returns is a promise, not the result of the operation. You can't return the result of an asynchronous process. More: How do I return the response from an asynchronous call?
In the upcoming ES2017 spec, though, we have syntactic sugar around promise consumption which will let you write this:
let data = await fetch('https://jsonplaceholder.typicode.com/users');
// --------^^^^^
console.log('we done, bruh!');
Note we don't even need the conditional, because await converts a promise rejection into an exception.
That code would need to be in an async function, e.g.:
(async function() {
let data = await fetch(/*...*/);
// use data here
})();
The JavaScript engines in some browsers already support async/await, but to use it in the wild, you'll want to transpile with Babel or similar.
Note: You've shown
so with a Promise, you might say:
let iWantToLogOut = function() {
let data = fetch('https://jsonplaceholder.typicode.com/users')
return new Promise((resolve) => {
resolve(data)
})
}
There are a couple of problems with that code:
It never settles the promise you created if the fetch fails.
It calls something data which is not data, it's a promise of data (that's mostly style, but it's misleading).
It exhibits the promise creation anti-pattern. You already have a promise (from fetch), no need to create another.
iWantToLogOut should be simply:
let iWantToLogOut = function() {
return fetch('https://jsonplaceholder.typicode.com/users');
};
That returns a promise that will be resolved with the data, or of course rejected. Which you'd then consume with promise methods or await (within an async function).
It is not a matter of easy.
Usually network calls should be handle asynchronously(I don't want to the anti-pattern of synchronous AJAX calls). At that point you have few options to handle it:
Callbacks
Promises
Observables
In you code above, when it's synchronous, the fetch should return immediately with a promise that will be resolve to the data only when the server has responded. Only then you can check the data for it's content. Further. Because every promise can be fulfilled or failed, in your then you can have a handler for each instead of using the ternary.
From the latest spec:
Synchronous XMLHttpRequest outside of workers is in the process of being removed from the web platform as it has detrimental effects to the end user’s experience. (This is a long process that takes many years.) Developers must not pass false for the async argument when current global object is a Window object. User agents are strongly encouraged to warn about such usage in developer tools and may experiment with throwing an InvalidAccessError exception when it occurs.
This is my first post so I hope I do it right. This is my first exposure to promises and 3 hours in I still can't get it. Our project does not have support for Async Wait, unfortunately. I'm using pouchdb over sqlite.
this.recipes is used to display a page of recipes. It returns as empty, however the first time, and then always lags one recipe behind after each recipe is added. I am pretty sure this is because I return this.recipeBook before all the promises are completed. I realize in the code I posted the final return is dangling outside of any promise, but when I put it in a promise I get a "type void is not assignable to type any[]" error in my RecipesPage.ts.
I appreciate any support you guys can offer and am excited to learn the right way to hand this, and thank you in advance.
This is my call in RecipesPage.ts:
this.recipes = this.database.getAllRecipes();
This is the method in database.service.ts:
public getAllRecipes() {
this.recipeBook = new Array<Recipe>();
this.recipeDatabase.allDocs
({
include_docs: true,
}).then(docs => {
this.objectArray = docs.rows.map(row => {
return row.doc;
})
}).then(function (){
for (let object of this.objectArray) {
this.myRecipe = this.convertObjectToRecipe(object);
this.recipeBook.push(this.myRecipe);
}
})
return this.recipeBook;
}
You don't. There's no real way to 'unpack' a promise like this. The closest you can realistically get is polling.
Instead, return the promise and call .then on the promise where you're waiting for it. This is ultimately what async and await are doing under the hood. Make decisions in the UI to choose how to render your content before its loaded (and as its being loaded).