Questions tagged [promise]

Promises are a tactic for deferred computing, suitable for several styles of concurrency: thread and event loop concurrency for local computation, and both synchronous and asynchronous remote messaging. A promise represents the eventual result of an asynchronous operation. The primary way of working with promises is through a method which registers transformations from the promise's eventual value or failure reason to a new promise.

In computer science, future, promise, and delay refer to constructs used for synchronizing in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is yet incomplete.

Promises are a common construct in javascript with new built in language support. There are several popular implementations of the concept such as bluebird and q. However, promises are not unique in JavaScript and similar patterns exist in many languages. Popular implementations exist in python, objective-c, c++, javascript, scala, dart, c# and most other languages. Some languages provide native language support for the construct.

Recurring questions:

Reading material:

Wikipedia: Futures and promises
You're missing the point of promises - an introduction to promises.
What are promises and why should I use them discusses the advantages of promises.
Why I am switching to promises - analysis of performance of promises and other patterns.
The promise Promises/A+ specification.
What is the deferred antipattern? - a verbose, error prone, but all too common way to use promises

Popular Implementations:

22514 questions

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Getting a UnhandledPromiseRejectionWarning when testing using mocha/chai

So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai: it('should transition with the correct event', (done) => { const cFSM = new CharacterFSM({}, emitter, transitions); …

javascript node.js promise mocha.js chai

asked Sep 27 '16 at 05:23

Jzop

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How to create an Observable from static data similar to http one in Angular?

I am having a service that has this method: export class TestModelService { public testModel: TestModel; constructor( @Inject(Http) public http: Http) { } public fetchModel(uuid: string = undefined): Observable { …

typescript promise angular observable rxjs

asked Feb 05 '16 at 08:43

Michail Michailidis

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Promise - is it possible to force cancel a promise

I use ES6 Promises to manage all of my network data retrieval and there are some situations where I need to force cancel them. Basically the scenario is such that I have a type-ahead search on the UI where the request is delegated to the backend has…

javascript promise cancellation

asked May 14 '15 at 09:03

Moonwalker

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Futures vs. Promises

I'm confusing myself with difference between a future and a promise. Obviously, they have different methods and stuff, but what is the actual use case? Is it?: when I'm managing some async task, I use future to get the value "in future" when I'm…

c++ c++11 promise future

asked Sep 27 '12 at 11:20

Šimon Tóth

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Break promise chain and call a function based on the step in the chain where it is broken (rejected)

Update: To help future viewers of this post, I created this demo of pluma's answer. Question: My goal seems fairly straightforward. step(1) .then(function() { return step(2); }, function() { stepError(1); return $q.reject(); }) …

javascript angularjs promise

asked Dec 21 '13 at 01:30

m59

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JavaScript array .reduce with async/await

Seem to be having some issues incorporating async/await with .reduce(), like so: const data = await bodies.reduce(async(accum, current, index) => { const methodName = methods[index] const method = this[methodName] if (methodName == 'foo') { …

javascript promise async-await reduce ecmascript-next

asked Dec 20 '16 at 13:26

benhowdle89

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Node JS Promise.all and forEach

I have an array like structure that exposes async methods. The async method calls return array structures that in turn expose more async methods. I am creating another JSON object to store values obtained from this structure and so I need to be…

javascript node.js asynchronous promise

asked Jul 14 '15 at 17:46

user3205931

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What is the equivalent of Bluebird Promise.finally in native ES6 promises?

Bluebird offers a finally method that is being called whatever happens in your promise chain. I find it very handy for cleaning purposes (like unlocking a resource, hiding a loader, ...) Is there an equivalent in ES6 native promises?

javascript promise bluebird

asked Mar 14 '16 at 22:10

Aric Lasry

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How to make a promise from setTimeout

This is not a realworld problem, I'm just trying to understand how promises are created. I need to understand how to make a promise for a function that returns nothing, like setTimeout. Suppose I have: function async(callback){ …

javascript settimeout promise

asked Mar 28 '14 at 08:18

laggingreflex

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17 answers

Cancel a vanilla ECMAScript 6 Promise chain

Is there a method for clearing the .thens of a JavaScript Promise instance? I've written a JavaScript test framework on top of QUnit. The framework runs tests synchronously by running each one in a Promise. (Sorry for the length of this code…

javascript promise cancellation es6-promise

asked Apr 06 '15 at 20:03

dx_over_dt

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Why does javascript ES6 Promises continue execution after a resolve?

As I understand a promise is something that can resolve() or reject() but I was suprised to find out that code in the promise continues to execute after a resolve or reject is called. I considered resolve or reject being an async-friendly version of…

javascript promise ecmascript-6 es6-promise

asked Mar 06 '15 at 09:55

Ludwig Van Beethoven

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How to use fetch in TypeScript

I am using window.fetch in Typescript, but I cannot cast the response directly to my custom type: I am hacking my way around this by casting the Promise result to an intermediate 'any' variable. What would be the correct method to do this? import {…

typescript promise fetch-api

asked Dec 12 '16 at 14:43

Kokodoko

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Why can I not throw inside a Promise.catch handler?

Why can't I just throw an Error inside the catch callback and let the process handle the error as if it were in any other scope? If I don't do console.log(err) nothing gets printed out and I know nothing about what happened. The process just…

javascript asynchronous promise throw es6-promise

asked Jun 08 '15 at 17:20

demian85

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Handling multiple catches in promise chain

I am still fairly new to promises and am using bluebird currently, however I have a scenario where I am not quite sure how to best deal with it. So for example I have a promise chain within an express app like…

javascript node.js promise bluebird

asked Sep 27 '14 at 16:02

Grofit

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Can promises have multiple arguments to onFulfilled?

I'm following the spec here and I'm not sure whether it allows onFulfilled to be called with multiple arguments. For example: promise = new Promise(function(onFulfilled, onRejected){ onFulfilled('arg1', 'arg2'); }) such that my…

javascript promise

asked Mar 31 '14 at 23:09

badunk

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