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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nodejs - How to promisify http.request? reject got called two times

I'm trying to wrap http.request into Promise: new Promise(function(resolve, reject) { var req = http.request({ host: '127.0.0.1', port: 4000, method: 'GET', path: '/api/v1/service' }, function(res) { …

node.js promise httprequest

asked Jul 22 '16 at 18:59

happy_marmoset

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Promise equivalent in C#

In Scala there is a Promise class that could be used to complete a Future manually. I am looking for an alternative in C#. I am writing a test and I want it to look it similar to this: // var MyResult has a field `Header` var promise = new…

c# promise async-await task future

asked Aug 17 '16 at 12:24

eddyP23

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How to specify resolution and rejection type of the promise in JSDoc?

I have some code that returns a promise object, e.g. using Q library for NodeJS. var Q = require('q'); /** * @returns ??? */ function task(err) { return err? Q.reject(new Error('Some error')) : Q.resolve('Some result'); } How to document…

javascript node.js documentation jsdoc promise

asked Oct 27 '12 at 21:37

Arikon

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fromPromise does not exist on type Observable

In Angular 2 using rxjs I was trying to convert a Promise to Observable. As many of online guides showed I used fromPromise on Observable. Which throws error: Property 'fromPromise' does not exist on type 'typeof Observable'. Observable was…

angular promise rxjs observable es6-promise

asked Aug 20 '17 at 17:16

Ahmad

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vue-router — Uncaught (in promise) Error: Redirected from "/login" to "/" via a navigation guard

Why is vue-router giving me this error? To be clear, the login flow works as intended but I want to a) get rid of the errro and b) understand why the error is happening. Error: Uncaught (in promise) Error: Redirected from "/login" to "/" via…

vue.js promise vue-router

asked Jun 05 '20 at 19:52

Kirk Ross

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Using promise function inside Javascript Array map

Having an array of objects [obj1, obj2] I want to use Map function to make a DB query (that uses promises) about all of them and attach the results of the query to each object. [obj1, obj2].map(function(obj){ …

javascript arrays promise

asked Sep 12 '16 at 13:57

Jorge

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How to handle the if-else in promise then?

In some case, when I get a return value from a promise object, I need to start two different then() precesses depend on the value's condition, like: promise().then(function(value){ if(//true) { // do something } else { // do…

node.js asynchronous promise

asked Oct 21 '15 at 10:52

Brick Yang

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Axios Interceptors retry original request and access original promise

I have an interceptor in place to catch 401 errors if the access token expires. If it expires it tries the refresh token to get a new access token. If any other calls are made during this time they are queued until the access token is…

javascript vue.js promise vuejs2 axios

asked Jul 27 '18 at 18:44

Tim Wickstrom

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8 answers

Replacing callbacks with promises in Node.js

I have a simple node module which connects to a database and has several functions to receive data, for example this function: dbConnection.js: import mysql from 'mysql'; const connection = mysql.createConnection({ host: 'localhost', user:…

javascript node.js promise

asked Feb 10 '15 at 13:03

Lior Erez

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How to add delay to promise inside then

fetch() { return axios.get('/rest/foo') //.then(response => {throw new Error(response)}) // Uncomment to test network error //.then( <> ) // Uncomment to simulate network delay } How do I add delay in the…

javascript asynchronous callback promise

asked Aug 15 '16 at 13:36

Tuomas Toivonen

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8 answers

Angular 2: Convert Observable to Promise

Q) How do I convert the following observable to a promise so I can call it with .then(...)? My method I want to convert to a promise: this._APIService.getAssetTypes().subscribe( assettypes => { …

typescript angular promise observable

asked Apr 21 '16 at 18:07

Dave

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6 answers

How do Clojure futures and promises differ?

Both futures and promises block until they have calculated their values, so what is the difference between them?

clojure terminology future promise

asked Jan 07 '11 at 07:51

yazz.com

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return value after a promise

I have a javascript function where I want to return the value that I get after the return method. Easier to see than explain function getValue(file){ var val; lookupValue(file).then(function(res){ val = res.val; } return…

javascript node.js promise q

asked Apr 09 '14 at 01:29

pedalpete

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9 answers

How can I execute array of promises in sequential order?

I have an array of promises that need to run in sequential order. var promises = [promise1, promise2, ..., promiseN]; Calling RSVP.all will execute them in parallel: RSVP.all(promises).then(...); But, how can I run them in sequence? I can…

javascript ember.js promise rsvp.js

asked Nov 20 '13 at 15:38

jaaksarv

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6 answers

How to cancel a HTTPRequest in Angular 2?

How to cancel a HTTPRequest in Angular 2? I know how to reject the request promise only. return new Promise((resolve, reject) => { this.currentLoading.set(url, {resolve, reject}); this.http.get(url, {headers: reqHeaders}) …

http promise angular

asked Apr 08 '16 at 03:19

tom10271

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