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Questions tagged [es6-promise]

An ES6 Promise is an ECMAScript 2015 object that represents an in-progress asynchronous operation

An ES6 Promise is an ECMAScript 2015 (ES6) object that represents the eventual completion or failure of an asynchronous operation. The ES6 Promise API is based on the Promises/A+ specification.

Consumer-side, a promise is essentially a returned object to attach callbacks to, instead of passing callbacks into a function. The .then function lets callers chain operations to be run subsequently.

In an ideal world, all asynchronous functions would return promises. However, a promise may also be created from scratch using its constructor, to wrap old callback-style function calls.

Promise objects have three possible states:

  • Pending
  • Fulfilled
  • Rejected

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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…
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…
Benjamin Gruenbaum
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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 =>…
Nathan Hagen
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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…
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?
theram
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How to reject in async/await syntax?

How can I reject a promise that returned by an async/await function? e.g. Originally: foo(id: string): Promise { return new Promise((resolve, reject) => { someAsyncPromise().then((value)=>resolve(200)).catch((err)=>reject(400)) …
Phoenix
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Handling errors in Promise.all

I have an array of Promises that I'm resolving with Promise.all(arrayOfPromises); I go on to continue the promise chain. Looks something like this existingPromiseChain = existingPromiseChain.then(function() { var arrayOfPromises =…
Jon
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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; //superfluous? } resolve(numerator / denominator); …
sam
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Axios get access to response header fields

I'm building a frontend app with React and Redux and I'm using axios to perform my requests. I would like to get access to all the fields in the header of the response. In my browser I can inspect the header and I can see that all the fields that I…
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What is an unhandled promise rejection?

For learning Angular 2, I am trying their tutorial. I am getting an error like this: (node:4796) UnhandledPromiseRejectionWarning: Unhandled promise rejection (r …
Mohammad Sadiqur Rahman
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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.resolve); return…
Thorben Croisé
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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 equivalent of running chained promises like…
Yanick Rochon
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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: 'settled' and 'resolved' 'settled' and…
jokeyrhyme
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What is the best way to limit concurrency when using ES6's Promise.all()?

I have some code that is iterating over a list that was queried out of a database and making an HTTP request for each element in that list. That list can sometimes be a reasonably large number (in the thousands), and I would like to make sure I am…
Chris
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JavaScript ES6 promise for loop

for (let i = 0; i < 10; i++) { const promise = new Promise((resolve, reject) => { const timeout = Math.random() * 1000; setTimeout(() => { console.log(i); }, timeout); }); // TODO: Chain this promise…
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