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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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

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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

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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

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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

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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

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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

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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

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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

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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

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Use async await with Array.map

Given the following code: var arr = [1,2,3,4,5]; var results: number[] = await arr.map(async (item): Promise => { await callAsynchronousOperation(item); return item + 1; }); which produces the following error: TS2322:…

javascript typescript promise async-await ecmascript-2017

asked Oct 19 '16 at 19:39

Alon

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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

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Resolve Javascript Promise outside the Promise constructor 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

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How to check 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

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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

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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 -

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