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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Is it possible to reject each Promise in Promise.allSettled if it takes more than 5 seconds to resolve?

I have a Promise.allSettled which i use to resolve data in the database. I have an array of Promises which I run through the Promise.allSettled, then I only use the resolved ones. Is it possible to set a timeout inside the Promise.allSettled, so…

javascript node.js promise async-await async.js

asked Dec 03 '20 at 10:09

TheStranger

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How would you display a promise object's state and value when overriding console.log

I'm attempting to display the console.log output in a div. I'm doing this by overriding the console.log function When I use the original console.log, a promise displays, in the console as (see 1. in code below): Promise { : "fulfilled",…

javascript html promise overriding console.log

asked Oct 27 '20 at 14:07

Lee

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1 answer

Cypress.io make async request in custom login command

I want to configure a custom login command in which I have to make a signIn call which returns a promise. commands.js: Cypress.Commands.add("login", () => { AuthLib.signIn(username, password).then((data) => { cy.setLocalStorage("accessToken",…

javascript testing promise async-await cypress

asked Oct 16 '20 at 15:46

xtra

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2 answers

Javascript: Awaiting resolved promise

I would like to know, if awaiting a resolved promise, will lead to a synchronous code execution or may lead to asynchronous one. I made this little snippet to check on browsers : const promise = new Promise((resolve) => { console.log('exec…

javascript promise async-await

asked Oct 15 '20 at 08:44

Poyoman

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1 answer

Async promise executor functions

I have a function that returns a Promise like this: async function processFolder(folder) { return new Promise(async (res) => { const table = {}; const list = await getHTMLlist(folder); if (list.length === 0) res(table); …

javascript promise async-await eslint

asked Oct 07 '20 at 14:47

Plastic

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2 answers

Subclassing the Promise built-in using extends

I want to add a cancel method to a subclass of the Promise built-in. Why does this not work? class CancellablePromise extends Promise { constructor(executor) { let cancel = null super((resolve,reject) => { cancel =…

javascript ecmascript-6 promise es6-class cancellation

asked Oct 01 '20 at 12:44

Ben Aston

53,718
65
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331

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2 answers

Possible to make an event handler wait until async / Promise-based code is done?

I am using the excellent Papa Parse library in nodejs mode, to stream a large (500 MB) CSV file of over 1 million rows, into a slow persistence API, that can only take one request at a time. The persistence API is based on Promises, but from Papa…

node.js promise es6-promise node-streams backpressure

asked Sep 05 '20 at 02:12

blitter

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1 answer

Empty then on promise

During work, I stumbled upon this line inside our codebase (designed to run in the browser). Is there any effect in this line? It seems unnecessary to me, but before removing it, I want to be absolutely sure that I am not overlooking some quirky…

javascript promise es6-promise

asked Sep 01 '20 at 15:37

Erik Brendel

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4 answers

How to use Promise.all to handle thousands of request

In my React app I have a component that send request to an online service that is capable to handle 50 requests max. I got a new request now to execute 7000 MAC’s. function App() { const [data, setData] = useState([]); useEffect(() => { const…

javascript reactjs promise axios

asked Aug 27 '20 at 15:05

angus

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0 answers

Type 'Promise' is missing the following properties from type 'ReactElement

I have a compilation issue when trying to render ClientDetails component passing a list of Client in props: I got this error message: Type '{ (): Promise; propTypes: {}; }' is not assignable to type 'ComponentClass |…

reactjs typescript promise es6-promise

asked Aug 16 '20 at 13:48

Mselmi Ali

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2 answers

Getting error TypeError: Promise.any is not a function

I am learning Promise.any from the following site. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/any The IDE does not show any error, but while running using yarn command , I am getting the following…

javascript node.js typescript promise

asked Jul 27 '20 at 21:07

Deba

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2 answers

React - useEffect with async setState causing additional re-renders

I have a React hook here export const useConfigDetails = (resourceName: string) => { const [isLoading, setLoading] = React.useState(true); const [error, setError] = React.useState(undefined); const [allEnvDetails,…

reactjs promise react-hooks

asked Jul 19 '20 at 08:41

Christie Chen

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1 answer

How Can I Unwrap an Array of Promises within a Chain of Array Methods?

I have a chain of several maps, one of which needs to perform a database operation for each array element, so I'm using async await. const resultsAsPromises = arr .map(syncDouble) .map(asyncMapper) This isn't a problem if it is the last item in…

javascript arrays promise method-chaining

asked Jul 16 '20 at 16:39

1252748

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1 answer

Typescript throws an error when calling Promise.all with an array of promises

I have a problem with Promises in Typescript. I'm trying to call Promise.all with an array of promises but instead, I get No overload matches this call. The last overload gave the following error. Argument of type '(Promise | Promise)[]' is not…

typescript promise

asked Jul 14 '20 at 12:14

Dominik

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2 answers

Handle Promise Catches in Typescript

I have a promise based API service that requests data from my backend. It also comes with it's own error catch, so I don't have to write it everywhere. When it's written like this: BackendService.ts ... getTasks() { return…

javascript typescript promise axios

asked Jul 12 '20 at 09:10

Vincent

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