Can I do async constructor() No, that’s a syntax error – just like constructor* (). A constructor is a method that doesn’t return anything (no promise, no generator), it only initialises the instance. And, if not how should a constructor work that does this Such a constructor should not exist at all, see Is it … Read more
JavaScript has the concept of “run to completion”. Unless an error is thrown, a function is executed until a return statement or its end is reached. Other code outside of the function can’t interfere with that (unless, again, an error is thrown). If you want resolve() to exit your initializer function, you have to prepend … Read more
There should be no side effect. It would be a browser bug if a non-referenced Promise in whatever state is keeping resources. Just make sure you don’t keep any reference to the Promise object and you’ll be fine. Beware that certain APIs such as setTimeout will keep a reference to the closure up to the … Read more
You could easily assign it to an exported variable, but you should not do that – the assignment happens asynchronously, and the variable might be read before that in the modules where it is imported. So instead, just export the promise1! // data.js import urls from ‘./urls’ import getData from ‘./get-data’ export default getData(urls).then(responses => … Read more
First off, you pretty much never want to write code that is a mix of callbacks and promises for async operations. If you’re moving to promises or introducing some promises, then you probably want to refactor the callbacks in that same section of code into promises. For the appropriate types of operations, there are so … Read more
I’ve knocked up a little helper function that you can re-use. Basically pass your promises as normal, and provide a callback to do what you want with the progress.. function allProgress(proms, progress_cb) { let d = 0; progress_cb(0); for (const p of proms) { p.then(()=> { d ++; progress_cb( (d * 100) / proms.length ); … Read more
UPDATE: As of rxjs 6.0.0-beta.3, operators and observable creators should be imported from rxjs. Furthermore, fromPromise is not part of the public API anymore and its wrapped in the from method. TL;DR; UPDATE For rxjs 6.0.0 use: import { from } from ‘rxjs’; var observableFromPromise = from(promiseSrc); UPDATE: After the release of the pipeable operators … Read more
Good question! The resolver passed to the promise constructor intentionally runs synchronous in order to support this use case: var deferreds = []; var p = new Promise(function(resolve, reject){ deferreds.push({resolve: resolve, reject: reject}); }); Then, at some later point in time: deferreds[0].resolve(“Hello”); // resolve the promise with “Hello” The reason the promise constructor is given … Read more
As georg said, you’re just not putting the promises in arr, because you never return anything from the anonymous function you’ve wrapped around new Promise. That function is also completely unnecessary, so: var arr = []; for (var i = 0; i < 2; i++) { arr.push(new Promise(function(resolve, reject) { setTimeout(function() { log(‘resolved !’); resolve(); … Read more
A promise is just an object with properties in Javascript. There’s no magic to it. So failing to resolve or reject a promise just fails to ever change the state from “pending” to anything else. This doesn’t cause any fundamental problem in Javascript because a promise is just a regular Javascript object. The promise will … Read more