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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7 answers

How can I fetch an array of URLs with Promise.all?

If I have an array of urls: var urls = ['1.txt', '2.txt', '3.txt']; // these text files contain "one", "two", "three", respectively. And I want to build an object that looks like this: var text = ['one', 'two', 'three']; I’ve been trying to learn…

javascript promise es6-promise fetch-api

asked Jul 29 '15 at 20:46

user2467065

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4 answers

Is it a good practice using Observable with async/await?

I am using angular 2 common http that return an Observable, but I face with a problem that my code likes a mesh when I use nested Observable call: this.serviceA.get().subscribe((res1: any) => { this.serviceB.get(res1).subscribe((res2: any) => { …

javascript angular typescript promise observable

asked Apr 01 '18 at 09:25

Thach Huynh

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5 answers

How to Check Whether an Angular $q promise Is Resolved

I understand that typically one would just attach continuation code with a then() call and chain behaviour when using promises. However, I want to kick off a promise-wrapped asynchronous call and then separately kick off a 3-second $timeout() so I…

angularjs promise q

asked Jul 26 '13 at 16:01

blaster

8,876
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1 answer

error TS2794: Expected 1 arguments, but got 0. Did you forget to include 'void' in your type argument to 'Promise'?

In my project, I was using Typescript@4.0.3 and it was working fine, but now I updated its version to latest Typescript@4.1.3 and it is giving me a lot of errors. I am unable to find anything in documentations and not getting any Idea how to resolve…

typescript promise resolve

asked Dec 18 '20 at 10:00

Naila Akbar

3,033
4
34
76

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2 answers

Deferred versus promise

What is the difference between Deferred and Promise other than the jQuery versions? What should I use for my need? I only want to call the fooExecute(). I only need the fooStart() and fooEnd() to toggle the html div status for example. //I'm using…

jquery jquery-deferred promise

asked Jun 25 '13 at 21:48

fletchsod

3,560
7
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2 answers

Chaining Observables in RxJS

I'm learning RxJS and Angular 2. Let's say I have a promise chain with multiple async function calls which depend on the previous one's result which looks like: var promiseChain = new Promise((resolve, reject) => { setTimeout(() => { …

javascript promise reactive-programming rxjs rxjs5

asked Jun 12 '16 at 07:42

Harindaka

4,658
8
43
62

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2 answers

How to support promises in Internet Explorer 11?

I have a simple code that runs perfectly on every browser except for the Internet Explorer 11. How can I make it work on all browsers? Codepen 'use strict'; let promise = new Promise((resolve, reject) => { setTimeout(() => { …

javascript internet-explorer promise internet-explorer-11 polyfills

asked Mar 15 '16 at 16:01

Billy Logan

2,470
6
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8 answers

Are nested promises normal in Node.js?

The problem I have been struggling with for two weeks now while learning Node.js is how to do synchronous programming using node. I found that no matter how I try to do things sequentially I always end up with nested promises. I have found that…

javascript node.js promise

asked Mar 04 '16 at 20:38

Grim

2,398
4
35
54

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6 answers

Use Promise to wait until polled condition is satisfied

I need to create a JavaScript Promise that will not resolve until a specific condition is true. Let's say I have a 3rd party library, and I need to wait until a certain data condition exists within that library. The scenario I am interested in is…

javascript promise

asked May 28 '15 at 11:57

Joseph Gabriel

8,339
3
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53

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4 answers

Error: Type is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor

I've written this function in TypeScript: export class LoginService { async isLoggedIn(): boolean { const r = await this.http.get('http://localhost:3000/api/user/isLoggedIn').toPromise(); return r.body; } } When I try to run the…

angular promise

asked Oct 08 '18 at 01:42

user275801

1,235
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13

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5 answers

How to catch uncaught exception in Promise

Is there any way to globally catch all exceptions including Promise exceptions. Example: window.onerror = function myErrorHandler(errorMsg, url, lineNumber) { alert("Error occured: " + errorMsg);//or any message return false; …

javascript exception promise

asked Jan 17 '15 at 16:39

John

2,494
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21
24

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4 answers

How to always run some code when a promise is fulfilled in Angular.js

In my Angular.js application, I'm running some asynchronous operation. Before it starts I cover the application with a modal div, then once the operation is complete, I need to remove the div, whether the operation was successful or not. Currently I…

javascript angularjs promise deferred finally

asked Apr 16 '13 at 14:52

laurent

88,262
77
290
428

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6 answers

How to debug javascript promises?

I am trying to understand how to debug asynchronous code that is based on promises. By Promises I mean ECMAScript 6 based promises and by debugging I mean using the built-in chrome or firefox debugger. What I am having trouble with - is that when an…

javascript promise es6-promise

asked Sep 13 '14 at 20:11

YemSalat

19,986
13
44
51

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14 answers

While loop with promises

What would be the idiomatic way to do something like a while loop with promises. So: do something if the condition still stands do it again repeat then do something else. dosomething.then(possilblydomoresomethings).then(finish) I've done it this…

node.js loops promise q

asked Jun 20 '13 at 15:28

Grummle

1,364
1
12
21

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10 answers

Is there a way to short circuit async/await flow?

All four functions are called below in update return promises. async function update() { var urls = await getCdnUrls(); var metadata = await fetchMetaData(urls); var content = await fetchContent(metadata); await render(content); …

javascript promise async-await cancellation ecmascript-next

asked Jun 03 '16 at 22:06

sbr

4,735
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