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

votes

3 answers

Unit-test promise-based code in Angularjs

I'm having hard times trying to test promise-based code in Angularjs. I have the following code in my controller: $scope.markAsDone = function(taskId) { tasksService.removeAndGetNext(taskId).then(function(nextTask) { …

javascript angularjs promise

asked Apr 18 '13 at 11:22

Michal Ostruszka

2,089
2
20
23

votes

4 answers

Angular JS: how to bind to promises

I am trying to bind a promise to a view. I don't know if you can do that directly, but that's what I'm attempting to do. Any ideas what I am doing wrong? Note: the source is a little contrived with the timeout and uses static data, but that's to…

angularjs promise

asked Oct 23 '12 at 14:45

Adam Tegen

25,378
33
125
153

votes

5 answers

Why is `.catch(err => console.error(err))` discouraged?

I'm using promises and have code that looks like the following: function getStuff() { return fetchStuff().then(stuff => process(stuff) ).catch(err => { console.error(err); }); } Or: async function getStuff() { try { const…

javascript node.js promise try-catch

asked Jun 17 '18 at 12:03

Benjamin Gruenbaum

270,886
87
504
504

votes

5 answers

fetch: Reject promise with JSON error object

I have an HTTP API that returns JSON data both on success and on failure. An example failure would look like this: ~ ◆ http get http://localhost:5000/api/isbn/2266202022 HTTP/1.1 400 BAD REQUEST Content-Length: 171 Content-Type:…

javascript promise es6-promise fetch-api

asked Apr 06 '15 at 14:38

jbaiter

6,913
4
30
40

votes

1 answer

Is there a way to tell if an ES6 promise is fulfilled/rejected/resolved?

I'm used to Dojo promises, where I can just do the following: promise.isFulfilled(); promise.isResolved(); promise.isRejected(); Is there a way to determine if an ES6 promise is fulfilled, resolved, or rejected? If not, is there a way to fill in…

javascript promise ecmascript-6

asked Jan 31 '14 at 16:50

knpwrs

15,691
12
62
103

votes

6 answers

Error handling in AngularJS http get then construct

How can I handle an HTTP error, e.g. 500, when using the AngularJS "http get then" construct (promises)? $http.get(url).then( function(response) { console.log('get',response) } ) Problem is, for any non 200 HTTP response, the inner…

javascript angularjs promise angular-promise angular-http

asked Jun 13 '13 at 05:58

Federico Elles

4,659
7
27
35

votes

2 answers

JavaScript naming convention for promises?

I feel it would be useful to have a naming convention for JavaScript variables which hold a promise. I don't generally like or advocate naming conventions beyond programming language standards, but in the style of programming where promises are…

javascript naming-conventions deferred promise

asked Jan 10 '13 at 21:12

jevakallio

35,324
3
105
112

votes

8 answers

using a fetch inside another fetch in javascript

I want to get an api and after that call another one. Is it wisely using a code like this in javascript? fetch(url, { method: 'get', }).then(function(response) { response.json().then(function(data) { …

javascript promise es6-promise fetch-api

asked Dec 05 '16 at 18:37

Dvlpr

1,439
1
11
21

votes

2 answers

How to check if a Promise is pending

I have this situation in which I would like to know what the status is of a promise. Below, the function start only calls someTest if it is not running anymore (Promise is not pending). The start function can be called many times, but if its called…

javascript promise ecmascript-6 es6-promise

asked Mar 29 '16 at 20:04

Jeanluca Scaljeri

26,343
56
205
333

votes

3 answers

What does this error mean — Uncaught TypeError: Already read?

In Javascript, when is this error thrown? index.js /** * Created by tushar.mathur on 24/12/15. */ 'use strict' const _ = require('lodash') const Rx = require('rx') const createDataStore = require('./src/createDataStore') const fetch = x =>…

javascript promise typeerror

asked Jan 14 '16 at 09:51

tusharmath

10,622
12
56
83

votes

4 answers

How to return from a Promise's catch/then block?

There are many tutorials on how to use "then" and "catch" while programming with JavaScript Promise. However, all these tutorials seem to miss an important point: returning from a then/catch block to break the Promise chain. Let's start with some…

javascript asynchronous promise

asked Aug 16 '15 at 06:48

lixiang

1,941
3
20
25

votes

5 answers

Catching Errors in JavaScript Promises with a First Level try ... catch

So, I want my first level catch to be the one that handles the error. Is there anyway to propagate my error up to that first catch? Reference code, not working (yet): Promise = require('./framework/libraries/bluebird.js'); function promise() { …

javascript node.js promise bluebird

asked Jul 27 '14 at 02:59

Kirk Ouimet

27,280
43
127
177

votes

1 answer

Returning an Axios Promise from function

Can someone please explain why returning an Axios promise allows for further chaining, but returning after applying a then()/catch() method does not? Example: const url = 'https://58f58f38c9deb71200ceece2.mockapi.io/Mapss' function…

javascript promise axios

asked Apr 18 '17 at 05:08

Matt Stone

3,705
4
23
40

votes

2 answers

Angular's $q.reject() vs deferred.reject()

I'm trying to get a handle on the Angular $q service and its related objects and APIs. When I look at the objects in my console I see: var deferred = $q.defer() ...(and then from console inspection)... $q: Object {defer: function, reject:…

javascript angularjs promise

asked Jun 27 '14 at 04:02

lostdorje

6,150
9
44
86

votes

4 answers

How to wait for a stream to finish piping? (Nodejs)

I have a for loop array of promises, so I used Promise.all to go through them and called then afterwards. let promises = []; promises.push(promise1); promises.push(promise2); promises.push(promise3); Promise.all(promises).then((responses) => { …

node.js asynchronous promise pipe pdftotext

asked Jun 15 '16 at 13:38

ThePumpkinMaster

2,181
5
22
31

Prev 1 2 3

…

99 100 Next