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:

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Understanding promises in Node.js

From what I have understood there are three ways of calling asynchronous code: Events, e.g. request.on("event", callback); Callbacks, e.g. fs.open(path, flags, mode, callback); Promises I found the node-promise library but I don’t get it. Could…

javascript node.js promise

asked Nov 28 '10 at 11:27

ajsie

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How to return many Promises and wait for them all before doing other stuff

I have a loop which calls a method that does stuff asynchronously. This loop can call the method many times. After this loop, I have another loop that needs to be executed only when all the asynchronous stuff is done. So this illustrates what I…

javascript asynchronous ecmascript-6 promise es6-promise

asked Jul 15 '15 at 09:36

Ganbin

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AngularJS : Where to use promises?

I saw some examples of Facebook Login services that were using promises to access FB Graph API. Example #1: this.api = function(item) { var deferred = $q.defer(); if (item) { facebook.FB.api('/' + item, function (result) { …

angularjs deferred promise

asked Mar 24 '13 at 21:24

Mak

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How do I await multiple promises in-parallel without 'fail-fast' behavior?

I'm using async/await to fire several api calls in parallel: async function foo(arr) { const results = await Promise.all(arr.map(v => { return doAsyncThing(v) })) return results } I know that, unlike loops, Promise.all executes…

javascript asynchronous promise async-await

asked Dec 22 '16 at 22:04

Brandon

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How to include multiple js files using jQuery $.getScript() method

I am trying to dynamically include javascript files into my js file. I did some research about it and find jQuery $.getScript() method would be a desired way to go. // jQuery $.getScript('/path/to/imported/script.js', function() { // script is…

javascript jquery html promise

asked Aug 03 '12 at 20:58

sozhen

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What is the difference between JavaScript promises and async await?

I have been using ECMAScript 6 and ECMAScript 7 features already (thanks to Babel) in my applications - both mobile and web. The first step obviously was to ECMAScript 6 levels. I learnt many async patterns, the promises (which are really…

javascript asynchronous promise async-await

asked Dec 21 '15 at 17:37

bozzmob

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Correct Try...Catch Syntax Using Async/Await

I like the flatness of the new Async/Await feature available in Typescript, etc. However, I'm not sure I like the fact that I have to declare the variable I'm awaiting on the outside of a try...catch block in order to use it later. Like so: let…

javascript promise async-await try-catch ecmascript-2017

asked Jun 20 '17 at 22:15

freedomflyer

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Promise : then vs then + catch

Is there any difference between the 2 followings codes ? myPromise.then(function() { console.log('success'); }).catch(function() { console.log('error'); }); myPromise.then(function() { console.log('success'); }, function() { …

javascript promise

asked Oct 22 '15 at 09:58

Magus

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Promise.resolve vs new Promise(resolve)

I'm using bluebird and I see two ways to resolve synchronous functions into a Promise, but I don't get the differences between both ways. It looks like the stacktrace is a little bit different, so they aren't just an alias, right? So what is the…

javascript promise bluebird

asked Nov 03 '14 at 09:27

Pipo

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Why is 'this' undefined inside class method when using promises?

I have a javascript class, and each method returns a Q promise. I want to know why this is undefined in method2 and method3. Is there a more correct way to write this code? function MyClass(opts){ this.options = opts; return this.method1() …

javascript node.js promise this q

asked Jan 21 '16 at 17:46

SteamDev

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TypeError: Cannot read property 'then' of undefined

loginService.islogged() Above function return a string like "failed". However, when I try to run then function on it, it will return error of TypeError: Cannot read property 'then' of undefined and the cursor is indicate right after connected and…

javascript angularjs promise

asked Jul 16 '14 at 18:34

ey dee ey em

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Can you resolve an angularjs promise before you return it?

I am trying to write a function that returns a promise. But there are times when the information requested is available immediately. I want to wrap it in a promise so that the consumer doesn't need to make a decision. function getSomething(id) { …

angularjs promise

asked Dec 12 '13 at 22:21

Craig Celeste

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Rethrowing error in promise catch

I found the following code in a tutorial: promise.then(function(result){ //some code }).catch(function(error) { throw(error); }); I'm a bit confused: does the catch call accomplish anything? It seems to me that it doesn't have any effect,…

javascript promise

asked Feb 10 '17 at 19:31

Tyler Durden

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Correct way to write loops for promise.

How to correctly construct a loop to make sure the following promise call and the chained logger.log(res) runs synchronously through iteration? (bluebird) db.getUser(email).then(function(res) { logger.log(res); }); // this is a promise I tried the…

javascript node.js promise bluebird

asked Jul 09 '14 at 17:24

user2127480

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Promises, pass additional parameters to then chain

A promise, just for example: var P = new Promise(function (resolve, reject) { var a = 5; if (a) { setTimeout(function(){ resolve(a); }, 3000); } else { reject(a); } }); After we call the .then() method on the…

javascript promise ecmascript-6 es6-promise

asked Oct 02 '15 at 17:15

user3110667

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