Full Answer
How many times can you use a JavaScript promise?
Resolved JavaScript Promises Can Be Used Multiple Times! In this post, App Dev Manager Ashley Grant shows how to cache data in JavaScript Promises. JavaScript Promises are powerful, and they become more powerful once we understand that a “resolved” Promise can be used more than once!
Is it possible to call resolve multiple times?
Calling resolve multiple times is a no-op*. What you want to do instead of using the language’s abstraction for value + time is to use the language’s abstraction for action + time – an async function (or just a function returning a promise) console.log("Resolving...");
Can you change a promise after it's been fulfilled?
Once they've gone from pending to fulfilled or from pending to rejected, they cannot be changed. So, you pretty much cannot and should not be using promises for something that you want to occur multiple times.
What are promises and how do they work?
They are essentially one-way state machines with three possible states pending, fulfilled and rejected. Once they've gone from pending to fulfilled or from pending to rejected, they cannot be changed. So, you pretty much cannot and should not be using promises for something that you want to occur multiple times.
What happens if you call then on its promise again?
How to change the return value of promise?
Can a promise be resolved once?
Can you resolve promises multiple times?
Is it safe to reject a promise multiple times?
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Can we resolve promise multiple times?
No. It is not safe to resolve/reject promise multiple times. It is basically a bug, that is hard to catch, becasue it can be not always reproducible.
Can you await a promise twice?
The implication of this is that promises can be used to memoize async computations. If you consume a promise whose result will be needed again later: consider holding on to the promise instead of its result! It's fine to await a promise twice, if you're happy to yield twice.
Can a promise be reused?
A promise cannot be 'reused'. It is succeeds (once), or it fails (once), or has not done either.
How many times promise can be rejected?
A Promise executor should call only one resolve or one reject . Once one state is changed (pending => fulfilled or pending => rejected), that's all. Any further calls to resolve or reject will be ignored.
Can we use async without await?
In this way, an async function without an await expression will run synchronously. If there is an await expression inside the function body, however, the async function will always complete asynchronously. Code after each await expression can be thought of as existing in a .then callback.
Is async-await slower than promises?
I found out that running async-await can be much slower in some scenarios. But if I click on the 'both' button, the 'await' version is ~3-4 times slower than the promises version.
Does await return a promise?
Inside an async function, you can use the await keyword before a call to a function that returns a promise. This makes the code wait at that point until the promise is settled, at which point the fulfilled value of the promise is treated as a return value, or the rejected value is thrown.
Are promises asynchronous?
A promise is used to handle the asynchronous result of an operation. JavaScript is designed to not wait for an asynchronous block of code to completely execute before other synchronous parts of the code can run. With Promises, we can defer the execution of a code block until an async request is completed.
How do I return a promise?
Promise resolve() method: If the value is a promise then promise is returned. If the value has a “then” attached to the promise, then the returned promise will follow that “then” to till the final state. The promise fulfilled with its value will be returned.
Is promise all synchronous?
Fulfillment. The returned promise is fulfilled with an array containing all the fulfilled values (including non-promise values) in the iterable passed as the argument. If an empty iterable is passed, then the promise returned by this method is fulfilled synchronously.
What if one promise fails in promise all?
// and then the same gets resolved. Example 4: As shown in this example, If one of the promises fails, then all the rest of the promises fail and result will be displayed in the console in the form of an Error. Then Promise. all() method gets rejected.
What happens if promise is not resolved?
A promise is just an object with properties in Javascript. There's no magic to it. So failing to resolve or reject a promise just fails to ever change the state from "pending" to anything else. This doesn't cause any fundamental problem in Javascript because a promise is just a regular Javascript object.
What happens if you await a resolved promise?
The expression is resolved in the same way as Promise. resolve() , which means thenable objects are supported, and if expression is not a promise, it's implicitly wrapped in a Promise and then resolved. If the promise is rejected, the await expression throws the rejected value.
How do you call multiple promises at once?
In this approach, we will use Promise. all() method which takes all promises in a single array as its input. As a result, this method executes all the promises in itself and returns a new single promise in which the values of all the other promises are combined together.
Does promise allSettled run in parallel?
allSettled(promises) lets you run promises in parallel and collect the statuses (either fulfilled or reject) into an aggregate array.
How do you handle an array of promises?
Wait for all promises to complete with Promise.all. Promise. all accepts an array of promises and returns a new promise that resolves only when all of the promises in the array have been resolved. ... Wait for at least one promise to complete with Promise.race. Promise. ... Wait for all promises to complete one-by-one.
How to resolve a promise multiple times? - Stack Overflow
It might sound weird, but I'm looking for a way to resolve a promise multiple times. Are there any approaches to make this possible? Think of the following example: getPromise() { const event =...
Resolving promise multiple times · GitHub
Context. I often meet the pattern where I need to either retrieve data from cache or fetch it from the server (both applies as well). I tried to implement such data loading policy in a generic fashion, but I cannot found an ideal design for this purpose, so I decided to share it and let the JS community help me to improve it :)
How to Wait for Multiple Promises to Resolve in JavaScript Using ...
Multiple Promises. This is good and fine, but what if we want to fetch two different accounts and only take action after both requests are resolved?. This is when we can use Promise.all!Promise.all takes an array of promises and only executes the function provided to then after all promises in the array resolve.. It will look something like this:
Resolved JavaScript Promises Can Be Used Multiple Times!
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Javascript - How to use promises with multiple .then()
You got it wrong. First promise.prototype.then resolve with just one Argument. Please check docs. And second thing promise return a promise, so you are just calling a query resolving there only, so no change in first resolved data and while chaining it will just pass the first data as it is
What is a promise in JavaScript?
A JavaScript Promise is more than just an object with a then function. It is, at its core, an object that represents some work to be run asynchronously. When creating a Promise, you pass its constructor an "executor" function with the asynchronous code in it. This executor function receives two functions as parameters.
Does a Promise run asynchronously?
I realize this code doesn't actually do anything asynchronous, but the code passed to the Promise does run asynchronously. The important thing for this post is that we are not limited to calling .then just once on a Promise. We can call it multiple times if we wish. Let's see what happens if we do just that.
Is the work in our promise asynchronous?
This is interesting! We get the same result from the Promise every time we call .then. But again, the work in our Promise isn't asynchronous, and I could just as easily have written this:
DavidBruant commented on Dec 27, 2016
A promise is the wrong tool for your need here. A Promise can only be fulfilled once. You need/want events here.
datrine commented on Apr 23, 2020
A promise should resolve only once, according to ECMAScript spec. However there's this concept of "multiple resolves" in NodeJs process API. I think I'll dig into that concept.
jasonk commented on Jun 24, 2020
FYI: The "multiple resolves" in the NodeJS process API is not an API that will allow you to resolve multiple times, it's an error event that gets emitted when you do.
What happens if you call then on its promise again?
If you call then(...)on its promise again, you immediately get the (first) resolved/rejected result.
How to change the return value of promise?
If you need to change the return value of promise, simply return new value in thenand chain next then/catchon it
Can a promise be resolved once?
I faced the same thing a while ago, indeed a promise can be only resolved once, another tries will do nothing (no error, no warning, no theninvocation).
Can you resolve promises multiple times?
1. There s no clear way to resolve promises multiple times because since it's resolved it's done. The better approach here is to use observer-observable pattern for example i wrote following code that observes socket client event. You can extend this code to met your need.
Is it safe to reject a promise multiple times?
No. It is not safe to resolve/reject promise multiple times. It is basically a bug, that is hard to catch, becasue it can be not always reproducible. There is pattern that can be used to trace such issues in debug time.
