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:

19807 questions

1750

votes

26 answers

Using async/await with a forEach loop

Are there any issues with using async/await in a forEach loop? I'm trying to loop through an array of files and await on the contents of each file. import fs from 'fs-promise' async function printFiles () { const files = await getFilePaths() //…

javascript node.js promise async-await ecmascript-2017

asked Jun 01 '16 at 18:55

Saad

32,434
19
55
98

1582

votes

30 answers

What is the difference between Promises and Observables?

What is the difference between Promise and Observable in Angular? An example on each would be helpful in understanding both the cases. In what scenario can we use each case?

angular promise rxjs angular-promise angular-observable

asked May 21 '16 at 15:43

Rohit

15,907
3
8
9

768

votes

21 answers

How do I convert an existing callback API to promises?

I want to work with promises but I have a callback API in a format like: 1. DOM load or other one time event: window.onload; // set to callback ... window.onload = function() { }; 2. Plain callback: function request(onChangeHandler) { …

javascript node.js callback promise bluebird

asked Mar 19 '14 at 22:47

Benjamin Gruenbaum

246,787
79
474
476

697

votes

17 answers

How do I access previous promise results in a .then() chain?

I have restructured my code to promises, and built a wonderful long flat promise chain, consisting of multiple .then() callbacks. In the end I want to return some composite value, and need to access multiple intermediate promise results. However the…

javascript scope promise bluebird es6-promise

asked Jan 31 '15 at 10:41

Bergi

513,640
108
821
1,164

653

votes

8 answers

Syntax for an async arrow function

I can mark a JavaScript function as "async" (i.e., returning a promise) with the async keyword. Like this: async function foo() { // Do something } What is the equivalent syntax for arrow functions?

javascript promise async-await arrow-functions

asked Mar 22 '17 at 22:50

BonsaiOak

20,455
5
25
50

563

votes

3 answers

What is the explicit promise construction antipattern and how do I avoid it?

I was writing code that does something that looks like: function getStuffDone(param) { | function getStuffDone(param) { var d = Q.defer(); /* or $q.defer */ | return new Promise(function(resolve, reject) { // or = new…

javascript promise q bluebird es6-promise

asked May 22 '14 at 10:07

Benjamin Gruenbaum

246,787
79
474
476

485

votes

11 answers

jQuery deferreds and promises - .then() vs .done()

I've been reading about jQuery deferreds and promises and I can't see the difference between using .then() & .done() for successful callbacks. I know Eric Hynds mentions that .done() and .success() map to the same functionality but I'm guessing so…

jquery promise jquery-deferred

asked Mar 25 '11 at 18:02

screenm0nkey

16,975
17
53
75

484

votes

7 answers

JavaScript Promises - reject vs. throw

I have read several articles on this subject, but it is still not clear to me if there is a difference between Promise.reject vs. throwing an error. For example, Using Promise.reject return asyncIsPermitted() .then(function(result) { if…

javascript promise

asked Oct 30 '15 at 21:48

Naresh

18,757
25
99
162

477

votes

19 answers

Wait until all promises complete even if some rejected

Let's say I have a set of Promises that are making network requests, of which one will fail: // http://does-not-exist will throw a TypeError var arr = [ fetch('index.html'), fetch('http://does-not-exist') ] Promise.all(arr) .then(res =>…

javascript promise es6-promise

asked Jul 15 '15 at 07:53

Nathan Hagen

10,750
4
24
31

464

votes

10 answers

Aren't promises just callbacks?

I've been developing JavaScript for a few years and I don't understand the fuss about promises at all. It seems like all I do is change: api(function(result){ api2(function(result2){ api3(function(result3){ // do work …

javascript callback promise q bluebird

asked Mar 20 '14 at 16:49

Benjamin Gruenbaum

246,787
79
474
476

406

votes

9 answers

What is std::promise?

I'm fairly familiar with C++11's std::thread, std::async and std::future components (e.g. see this answer), which are straight-forward. However, I cannot quite grasp what std::promise is, what it does and in which situations it is best used. The…

c++ multithreading c++11 promise standard-library

asked Jun 12 '12 at 20:23

Kerrek SB

428,875
83
813
1,025

394

votes

17 answers

How do I tell if an object is a Promise?

Whether it's an ES6 Promise or a bluebird Promise, Q Promise, etc. How do I test to see if a given object is a Promise?

javascript promise q bluebird es6-promise

asked Jan 02 '15 at 17:47

theram

4,435
2
11
14

351

votes

22 answers

Resolve Javascript Promise outside function scope

I have been using ES6 Promise. Ordinarily, a Promise is constructed and used like this new Promise(function(resolve, reject){ if (someCondition){ resolve(); } else { reject(); } }); But I have been doing something like…

javascript promise es6-promise

asked Oct 01 '14 at 20:40

Morio

6,142
3
20
26

339

votes

5 answers

What's the difference between returning value or Promise.resolve from then()

What is the difference between: new Promise(function(res, rej) { res("aaa"); }) .then(function(result) { return "bbb"; }) .then(function(result) { console.log(result); }); and this: new Promise(function(res, rej)…

javascript angularjs promise q

asked Dec 31 '14 at 02:08

spirytus

9,438
12
52
74

330

votes

29 answers

Resolve promises one after another (i.e. in sequence)?

Consider the following code that reads an array of files in a serial/sequential manner. readFiles returns a promise, which is resolved only once all files have been read in sequence. var readFile = function(file) { ... // Returns a…

javascript promise q sequential serial-processing

asked Jul 05 '14 at 11:48

XåpplI'-I0llwlg'I -

18,987
23
96
146

2 3

…

99 100 Next