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.
promise
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answers
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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 ...
- 51.8k
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answers
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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?
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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 ...
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votes
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answers
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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 ...
- 274k
754
votes
17
answers
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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 ...
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8
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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 ...
- 24.9k
652
votes
3
answers
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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) {
return new Promise(function(resolve, reject) {
myPromiseFn(param+1)
.then(function(val) {
...
- 274k
584
votes
20
answers
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views
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-...
- 12.6k
549
votes
10
answers
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views
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<number> => {
await callAsynchronousOperation(item);
return item +...
- 11.2k
524
votes
12
answers
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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()...
- 18.6k
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votes
29
answers
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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();...
- 8,501
519
votes
19
answers
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views
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?
- 5,731
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10
answers
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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){
...
- 274k
456
votes
9
answers
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views
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 ...
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36
answers
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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 = ...
- 22.1k
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votes
6
answers
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Do I need to return after early resolve/reject?
Suppose I have the following code.
function divide(numerator, denominator) {
return new Promise((resolve, reject) => {
if(denominator === 0){
reject("Cannot divide by 0");
return; //...
- 4,153
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votes
6
answers
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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"; // directly returning string
})
.then(function(result) {
...
- 10.9k
383
votes
9
answers
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views
What's the difference between a Future and a Promise?
What's the difference between Future and Promise?
They both act like a placeholder for future results, but where is the main difference?
- 18.3k
337
votes
15
answers
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Axios handling errors
I'm trying to understand javascript promises better with Axios. What I pretend is to handle all errors in Request.js and only call the request function from anywhere without having to use catch().
In ...
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votes
5
answers
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What are the differences between Deferred, Promise and Future in JavaScript?
What are the differences between Deferreds, Promises and Futures?
Is there a generally approved theory behind all these three?
- 101k
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Promise.all: Order of resolved values
Looking at MDN it looks like the values passed to the then() callback of Promise.all contains the values in the order of the promises. For example:
var somePromises = [1, 2, 3, 4, 5].map(Promise....
- 12.6k
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votes
14
answers
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Is Node.js native Promise.all processing in parallel or sequentially?
I would like to clarify this point, as the documentation is not too clear about it;
Q1: Is Promise.all(iterable) processing all promises sequentially or in parallel? Or, more specifically, is it the ...
- 52.5k
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votes
9
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Why is my asynchronous function returning Promise { <pending> } instead of a value?
My code:
let AuthUser = data => {
return google.login(data.username, data.password).then(token => { return token } )
}
And when i try to run something like this:
let userToken = AuthUser(...
- 5,372
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votes
14
answers
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views
How can I access the value of a promise?
I'm looking at this example from Angular's documentation for $q, but I think this probably applies to promises in general. The example below is copied verbatim from their documentation with their ...
262
votes
6
answers
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views
Difference between microtask and macrotask within an event loop context
I've just finished reading the Promises/A+ specification and stumbled upon the terms microtask and macrotask: see http://promisesaplus.com/#notes
I've never heard of these terms before, and now I'm ...
- 7,489
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votes
31
answers
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How can I synchronously determine a JavaScript Promise's state?
I have a pure JavaScript Promise (built-in implementation or poly-fill):
var promise = new Promise(function (resolve, reject) { /* ... */ });
From the specification, a Promise can be one of:
'...
- 3,438
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votes
1
answer
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Are there still reasons to use promise libraries like Q or BlueBird now that we have ES6 promises? [closed]
After Node.js added native support for promises, are there still reasons to use libraries like Q or BlueBird?
For example if you are starting a new project and let's assume in this project you don't ...
- 11.5k
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votes
6
answers
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Why does .json() return a promise, but not when it passes through .then()?
I've been messing around with the fetch() api recently, and noticed something which was a bit quirky.
let url = "http://jsonplaceholder.typicode.com/posts/6";
let iterator = fetch(url);
iterator
...
- 2,569
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votes
8
answers
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How to unwrap the type of a Promise?
Say I have the following code:
async promiseOne() {
return 1
} // => Promise<number>
const promisedOne = promiseOne()
typeof promisedOne // => Promised<number>
How would I go ...
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votes
7
answers
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When is .then(success, fail) considered an antipattern for promises?
I had a look at the bluebird promise FAQ, in which it mentions that .then(success, fail) is an antipattern. I don't quite understand its explanation as for the try and catch.
What's wrong with the ...
- 4,691
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votes
8
answers
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Return from a promise then()
I have got a javascript code like this:
function justTesting() {
promise.then(function(output) {
return output + 1;
});
}
var test = justTesting();
I have got always an undefined value for ...
- 2,373
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votes
6
answers
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views
How do I promisify native XHR?
I want to use (native) promises in my frontend app to perform XHR request but without all the tomfoolery of a massive framework.
I want my xhr to return a promise but this doesn't work (giving me: ...
- 39.2k
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votes
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answers
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How to find which promises are unhandled in Node.js UnhandledPromiseRejectionWarning?
Node.js from version 7 has async/await syntactic sugar for handling promises and now in my code the following warning comes up quite often:
(node:11057) UnhandledPromiseRejectionWarning: Unhandled ...
- 2,771
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How do I wait for a promise to finish before returning the variable of a function?
I'm still struggling with promises, but making some progress thanks to the community here.
I have a simple JS function which queries a Parse database. It's supposed to return the array of results, ...
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5
answers
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views
Is it bad practice to have a constructor function return a Promise?
I'm trying to create a constructor for a blogging platform and it has many async operations going on inside. These range from grabbing the posts from directories, parsing them, sending them through ...
- 5,779
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votes
8
answers
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views
How to pass parameter to a promise function
this might seem a silly question but I am a newbie in this topic. I am working on promises on node js. And I want to pass parameter to a promise function. However I could not figure it out.
...
- 2,120
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answers
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views
How to cancel an $http request in AngularJS?
Given a Ajax request in AngularJS
$http.get("/backend/").success(callback);
what is the most effective way to cancel that request if another request is launched (same backend, different parameters ...
- 20.2k
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votes
8
answers
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views
Using setTimeout on promise chain
Here i am trying to wrap my head around promises.Here on first request i fetch a set of links.and on next request i fetch the content of first link.But i want to make a delay before returning next ...
- 9,014
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votes
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How to wait for a JavaScript Promise to resolve before resuming function?
I'm doing some unit testing. The test framework loads a page into an iFrame and then runs assertions against that page. Before each test begins, I create a Promise which sets the iFrame's onload ...
- 13.8k
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votes
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answers
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How to promisify Node's child_process.exec and child_process.execFile functions with Bluebird?
I'm using the Bluebird promise library under Node.js, it's great! But I have a question:
If you take a look at the documentation of Node's child_process.exec and child_process.execFile you can see ...
- 2,691
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votes
13
answers
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Promise - is it possible to force cancel a promise
I use ES6 Promises to manage all of my network data retrieval and there are some situations where I need to force cancel them.
Basically the scenario is such that I have a type-ahead search on the UI ...
- 3,582
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votes
6
answers
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Angular HttpPromise: difference between `success`/`error` methods and `then`'s arguments
According to AngularJS doc, calls to $http return the following:
Returns a promise object with the standard then method and two http specific methods: success and error. The then method takes two ...
- 5,161
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votes
3
answers
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Placement of catch BEFORE and AFTER then
I have trouble understanding the difference between putting .catch BEFORE and AFTER then in a nested promise.
Alternative 1:
test1Async(10).then((res) => {
return test2Async(22)
.then((res) ...
- 4,458
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votes
23
answers
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views
typescript: error TS2693: 'Promise' only refers to a type, but is being used as a value here
I am trying to use Typescript for my AWS Lambda and i am getting the following errors where ever I use promises.
error TS2693: 'Promise' only refers to a type, but is being used as a value here.
I ...
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How do I properly test promises with mocha and chai?
The following test is behaving oddly:
it('Should return the exchange rates for btc_ltc', function(done) {
var pair = 'btc_ltc';
shapeshift.getRate(pair)
.then(function(data){
...
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6
answers
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views
How to create an Observable from static data similar to http one in Angular?
I am having a service that has this method:
export class TestModelService {
public testModel: TestModel;
constructor( @Inject(Http) public http: Http) {
}
public fetchModel(uuid: ...
172
votes
5
answers
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Returning Promises from Vuex actions
I recently started migrating things from jQ to a more structured framework being VueJS, and I love it!
Conceptually, Vuex has been a bit of a paradigm shift for me, but I'm confident I know what its ...
- 3,963
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8
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Getting a UnhandledPromiseRejectionWarning when testing using mocha/chai
So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai:
it('should transition with the correct event', (done) => {
const ...
- 1,735
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votes
5
answers
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How to use fetch in TypeScript
I am using window.fetch in Typescript, but I cannot cast the response directly to my custom type:
I am hacking my way around this by casting the Promise result to an intermediate 'any' variable.
...
- 27.2k
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votes
11
answers
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JavaScript array .reduce with async/await
Seem to be having some issues incorporating async/await with .reduce(), like so:
const data = await bodies.reduce(async(accum, current, index) => {
const methodName = methods[index]
const ...
- 37.2k