This my road of growing up: callbacks, promises, async/await. I keep in mind a pattern - fluent builder that looks more to interpreter. I would like to mix async/await with interpreter. Is it possible? I feel that it is not possible. But I can define why exactly.
I have:
after(async () => {
await shellContainer.uninstall()
await shellContainer.dispose()
})
I am interesting in:
after(async () => {
await shellContainer
.uninstall()
.dispose()
})
Regards.
To separate concerns, you could introduce a dedicated builder which implements the fluent interface.
With that you only need to return a promise from the final build method, making things a lot simpler.
The following is a rather crude but functional example:
class ShellContainer
{
uninstall() {
return new Promise(resolve => {
setTimeout(() => {
console.log('uninstalled');
resolve();
}, 400 + Math.random() * 600);
});
}
dispose() {
return new Promise(resolve => {
setTimeout(() => {
console.log('disposed');
resolve();
}, 400 + Math.random() * 600);
});
}
}
class ShellContainerBuilder
{
container;
plan;
constructor() {
this.reset();
}
reset() {
this.container = new ShellContainer();
this.plan = () => Promise.resolve();
}
stage(op) {
this.plan = ((prev) => () => prev().then(op))(this.plan);
}
uninstall() {
this.stage(() => this.container.uninstall());
return this;
}
dispose() {
this.stage(() => this.container.dispose());
return this;
}
build() {
return this.plan().then(() => this.container);
}
}
(async () => {
console.log('starting w/o builder:');
const shellContainer1 = new ShellContainer();
await shellContainer1.uninstall();
await shellContainer1.dispose();
console.log('w/o builder done.');
console.log('starting w/ builder:');
const shellContainer2 = await (new ShellContainerBuilder()).uninstall().dispose().build();
console.log(shellContainer2);
console.log('w/ builder done.');
})();
If you change .uninstall to return the instance (shellContainer) while assigning its Promise to a property of the instance, and retrieving that Promise in the chained call, yes, it's possible:
class ShellContainer {
uninstall() {
// Chain or construct a Promise and assign the result to a property of the instance:
this.uninstProm = new Promise((resolve, reject) => {
// do stuff
});
return this;
}
dispose() {
return this.uninstProm.then(() => {
// do stuff
});
}
}
and then,
await shellContainer
.uninstall()
.dispose()
will resolve once dispose finishes.
Note that with this approach, calling uninstall by itself may result in unexpected behavior, since .uninsall will be returning the instance synchronously, rather than the Promise. You might consider an extra argument or something to indicate whether you want to chain the uninstall call with something else, or whether you want the Promise to be returned directly, perhaps something like
class ShellContainer {
uninstall(doChain) {
// Chain or construct a Promise and assign the result to a property of the instance:
this.uninstProm = new Promise((resolve, reject) => {
// do stuff
});
return doChain ? this : this.uninstProm;
}
dispose() {
return this.uninstProm.then(() => {
// do stuff
});
}
}
and
await shellContainer
.uninstall(true)
.dispose()
or just
await shellContainer.uninstall(); // no doChain argument
But if there's only going to be one Promise, there's not much need for await in many cases - it may not make the code clearer at all. For example
after(async () => {
await shellContainer
.uninstall()
.dispose()
})
is equivalent to
after(() => shellContainer
.uninstall()
.dispose()
);
The pattern - Interpreter that implements Fluent builder is not suitable for Async/Await. Because await is applied per an operation. There is not such syntax to process async in the middle of the chain. To handle async operations you should split chaining and wrap the operations by await operator like it is at the first code snippet.
It might sound weird, but I'm looking for a way to resolve a promise multiple times. Are there any approaches to make this possible?
Think of the following example:
getPromise() {
const event = new Event('myEvent');
setTimeout(() => {
window.dispatchEvent(event);
}, 5000);
setTimeout(() => {
window.dispatchEvent(event);
}, 7000);
return new Promise((resolve) => {
window.addEventListener('myEvent', () => {
resolve('some value'));
});
resolve('some value'));
});
};
And then .then():
getPromise().then(data => {console.log(data)})
Should give the following result:
some value // initial
some value // after 5000ms
some value // after 7000ms
So I know there are libraries to stream data, but I'm really looking for a native non-callbak approach to achieve this.
How to resolve a promise multiple times?
You can't. Promises can only be resolved once. Once they have been resolved, they never ever change their state again. They are essentially one-way state machines with three possible states pending, fulfilled and rejected. Once they've gone from pending to fulfilled or from pending to rejected, they cannot be changed.
So, you pretty much cannot and should not be using promises for something that you want to occur multiple times. Event listeners or observers are a much better match than promises for something like that. Your promise will only ever notify you about the first event it receives.
I don't know why you're trying to avoid callbacks in this case. Promises use callbacks too in their .then() handlers. You will need a callback somewhere to make your solution work. Can you explain why you don't just use window.addEventListener('myEvent', someCallback) directly since that will do what you want?
You could return a promise-like interface (that does not follow Promise standards) that does call its notification callbacks more than once. To avoid confusion with promises, I would not use .then() as the method name:
function getNotifier() {
const event = new Event('myEvent');
setTimeout(() => {
window.dispatchEvent(event);
}, 500);
setTimeout(() => {
window.dispatchEvent(event);
}, 700);
let callbackList = [];
const notifier = {
notify: function(fn) {
callbackList.push(fn);
}
};
window.addEventListener('myEvent', (data) => {
// call all registered callbacks
for (let cb of callbackList) {
cb(data);
}
});
return notifier;
};
// Usage:
getNotifier().notify(data => {console.log(data.type)})
I have a solution in Typescript.
export class PromiseParty {
private promise: Promise<string>;
private resolver: (value?: string | PromiseLike<string>) => void;
public getPromise(): Promise<string> {
if (!this.promise) {
this.promise = new Promise((newResolver) => { this.resolver = newResolver; });
}
return this.promise;
}
public setPromise(value: string) {
if(this.resolver) {
this.resolver(value);
this.promise = null;
this.resolver = null;
}
}
}
export class UseThePromise {
public constructor(
private promiseParty: PromiseParty
){
this.init();
}
private async init(){
const subscribe = () => {
const result = await this.promiseParty.getPromise();
console.log(result);
subscribe(); //resubscribe!!
}
subscribe(); //To start the subscribe the first time
}
}
export class FeedThePromise {
public constructor(
private promiseParty: PromiseParty
){
setTimeout(() => {
this.promiseParty.setPromise("Hello");
}, 1000);
setTimeout(() => {
this.promiseParty.setPromise("Hello again!");
}, 2000);
setTimeout(() => {
this.promiseParty.setPromise("Hello again and again!");
}, 3000);
}
}
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 below to take the resolve outside for the sake of flexibility.
var outsideResolve;
var outsideReject;
new Promise(function(resolve, reject) {
outsideResolve = resolve;
outsideReject = reject;
});
And later
onClick = function(){
outsideResolve();
}
This works fine, but is there an easier way to do this? If not, is this a good practice?
simple:
var promiseResolve, promiseReject;
var promise = new Promise(function(resolve, reject){
promiseResolve = resolve;
promiseReject = reject;
});
promiseResolve();
Bit late to the party here, but another way to do it would be to use a Deferred object. You essentially have the same amount of boilerplate, but it's handy if you want to pass them around and possibly resolve outside of their definition.
Naive Implementation:
class Deferred {
constructor() {
this.promise = new Promise((resolve, reject)=> {
this.reject = reject
this.resolve = resolve
})
}
}
function asyncAction() {
var dfd = new Deferred()
setTimeout(()=> {
dfd.resolve(42)
}, 500)
return dfd.promise
}
asyncAction().then(result => {
console.log(result) // 42
})
ES5 Version:
function Deferred() {
var self = this;
this.promise = new Promise(function(resolve, reject) {
self.reject = reject
self.resolve = resolve
})
}
function asyncAction() {
var dfd = new Deferred()
setTimeout(function() {
dfd.resolve(42)
}, 500)
return dfd.promise
}
asyncAction().then(function(result) {
console.log(result) // 42
})
No, there is no other way to do this - the only thing I can say is that this use case isn't very common. Like Felix said in the comment - what you do will consistently work.
It's worth mentioning that the reason the promise constructor behaves this way is throw safety - if an exception you did not anticipate happens while your code is running inside the promise constructor it will turn into a rejection, this form of throw safety - converting thrown errors to rejections is important and helps maintain predictable code.
For this throw safety reason, the promise constructor was chosen over deferreds (which are an alternative promise construction way that do allow what you're doing) - as for best practices - I'd pass the element and use the promise constructor instead:
var p = new Promise(function(resolve, reject){
this.onclick = resolve;
}.bind(this));
For this reason - whenever you can use the promise constructor over exporting the functions - I recommend you do use it. Whenever you can avoid both - avoid both and chain.
Note, that you should never use the promise constructor for things like if(condition), the first example could be written as:
var p = Promise[(someCondition)?"resolve":"reject"]();
I liked #JonJaques answer but I wanted to take it a step further.
If you bind then and catch then the Deferred object, then it fully implements the Promise API and you can treat it as promise and await it and such.
⚠️ Editor's Note: I don't recommend this kind of pattern anymore since at the time of writing, Promise.prototype.finally was not a thing yet, then it became a thing… This could happen to other methods so I recommend you augment the promise instance with resolve and reject functions instead:
function createDeferredPromise() {
let resolve
let reject
const promise = new Promise((thisResolve, thisReject) => {
resolve = thisResolve
reject = thisReject
})
return Object.assign(promise, {resolve, reject})
}
Go upvote someone else's answer.
class DeferredPromise {
constructor() {
this._promise = new Promise((resolve, reject) => {
// assign the resolve and reject functions to `this`
// making them usable on the class instance
this.resolve = resolve;
this.reject = reject;
});
// bind `then` and `catch` to implement the same interface as Promise
this.then = this._promise.then.bind(this._promise);
this.catch = this._promise.catch.bind(this._promise);
this.finally = this._promise.finally.bind(this._promise);
this[Symbol.toStringTag] = 'Promise';
}
}
const deferred = new DeferredPromise();
console.log('waiting 2 seconds...');
setTimeout(() => {
deferred.resolve('whoa!');
}, 2000);
async function someAsyncFunction() {
const value = await deferred;
console.log(value);
}
someAsyncFunction();
A solution I came up with in 2015 for my framework. I called this type of promises Task
function createPromise(handler){
var resolve, reject;
var promise = new Promise(function(_resolve, _reject){
resolve = _resolve;
reject = _reject;
if(handler) handler(resolve, reject);
})
promise.resolve = resolve;
promise.reject = reject;
return promise;
}
// create
var promise = createPromise()
promise.then(function(data){ alert(data) })
// resolve from outside
promise.resolve(200)
Accepted answer is wrong. It's pretty easy using scope and references, though it may make Promise purists angry:
const createPromise = () => {
let resolver;
return [
new Promise((resolve, reject) => {
resolver = resolve;
}),
resolver,
];
};
const [ promise, resolver ] = createPromise();
promise.then(value => console.log(value));
setTimeout(() => resolver('foo'), 1000);
We are essentially grabbing the reference to the resolve function when the promise is created, and we return that so it can be set externally.
In one second the console will output:
> foo
A helper method would alleviate this extra overhead, and give you the same jQuery feel.
function Deferred() {
let resolve;
let reject;
const promise = new Promise((res, rej) => {
resolve = res;
reject = rej;
});
return { promise, resolve, reject };
}
Usage would be
const { promise, resolve, reject } = Deferred();
displayConfirmationDialog({
confirm: resolve,
cancel: reject
});
return promise;
Which is similar to jQuery
const dfd = $.Deferred();
displayConfirmationDialog({
confirm: dfd.resolve,
cancel: dfd.reject
});
return dfd.promise();
Although, in a use case this simple, native syntax is fine
return new Promise((resolve, reject) => {
displayConfirmationDialog({
confirm: resolve,
cancel: reject
});
});
I'm using a helper function to create what I call a "flat promise" -
function flatPromise() {
let resolve, reject;
const promise = new Promise((res, rej) => {
resolve = res;
reject = rej;
});
return { promise, resolve, reject };
}
And I'm using it like so -
function doSomethingAsync() {
// Get your promise and callbacks
const { resolve, reject, promise } = flatPromise();
// Do something amazing...
setTimeout(() => {
resolve('done!');
}, 500);
// Pass your promise to the world
return promise;
}
See full working example -
function flatPromise() {
let resolve, reject;
const promise = new Promise((res, rej) => {
resolve = res;
reject = rej;
});
return { promise, resolve, reject };
}
function doSomethingAsync() {
// Get your promise and callbacks
const { resolve, reject, promise } = flatPromise();
// Do something amazing...
setTimeout(() => {
resolve('done!');
}, 500);
// Pass your promise to the world
return promise;
}
(async function run() {
const result = await doSomethingAsync()
.catch(err => console.error('rejected with', err));
console.log(result);
})();
Edit:
I have created an NPM package called flat-promise and the code is also available on GitHub.
Just in case somebody came looking for a typescript version of a util simplifying this task:
export const deferred = <T>() => {
let resolve!: (value: T | PromiseLike<T>) => void;
let reject!: (reason?: any) => void;
const promise = new Promise<T>((res, rej) => {
resolve = res;
reject = rej;
});
return {
resolve,
reject,
promise,
};
};
This can be used eg. like:
const {promise, resolve} = deferred<string>();
promise.then((value) => console.log(value)); // nothing
resolve('foo'); // console.log: foo
You can wrap the Promise in a class.
class Deferred {
constructor(handler) {
this.promise = new Promise((resolve, reject) => {
this.reject = reject;
this.resolve = resolve;
handler(resolve, reject);
});
this.promise.resolve = this.resolve;
this.promise.reject = this.reject;
return this.promise;
}
promise;
resolve;
reject;
}
// How to use.
const promise = new Deferred((resolve, reject) => {
// Use like normal Promise.
});
promise.resolve(); // Resolve from any context.
I find myself missing the Deferred pattern as well in certain cases. You can always create one on top of a ES6 Promise:
export default class Deferred<T> {
private _resolve: (value: T) => void = () => {};
private _reject: (value: T) => void = () => {};
private _promise: Promise<T> = new Promise<T>((resolve, reject) => {
this._reject = reject;
this._resolve = resolve;
})
public get promise(): Promise<T> {
return this._promise;
}
public resolve(value: T) {
this._resolve(value);
}
public reject(value: T) {
this._reject(value);
}
}
Many of the answers here are similar to the last example in this article.
I am caching multiple Promises, and the resolve() and reject() functions can be assigned to any variable or property. As a result I am able to make this code slightly more compact:
function defer(obj) {
obj.promise = new Promise((resolve, reject) => {
obj.resolve = resolve;
obj.reject = reject;
});
}
Here is a simplified example of using this version of defer() to combine a FontFace load Promise with another async process:
function onDOMContentLoaded(evt) {
let all = []; // array of Promises
glob = {}; // global object used elsewhere
defer(glob);
all.push(glob.promise);
// launch async process with callback = resolveGlob()
const myFont = new FontFace("myFont", "url(myFont.woff2)");
document.fonts.add(myFont);
myFont.load();
all.push[myFont];
Promise.all(all).then(() => { runIt(); }, (v) => { alert(v); });
}
//...
function resolveGlob() {
glob.resolve();
}
function runIt() {} // runs after all promises resolved
Update: 2 alternatives in case you want to encapsulate the object:
function defer(obj = {}) {
obj.promise = new Promise((resolve, reject) => {
obj.resolve = resolve;
obj.reject = reject;
});
return obj;
}
let deferred = defer();
and
class Deferred {
constructor() {
this.promise = new Promise((resolve, reject) => {
this.resolve = resolve;
this.reject = reject;
});
}
}
let deferred = new Deferred();
Our solution was to use closures to store the resolve/reject functions and additionally attach a function to extend the promise itself.
Here is the pattern:
function getPromise() {
var _resolve, _reject;
var promise = new Promise((resolve, reject) => {
_reject = reject;
_resolve = resolve;
});
promise.resolve_ex = (value) => {
_resolve(value);
};
promise.reject_ex = (value) => {
_reject(value);
};
return promise;
}
And using it:
var promise = getPromise();
promise.then(value => {
console.info('The promise has been fulfilled: ' + value);
});
promise.resolve_ex('hello');
// or the reject version
//promise.reject_ex('goodbye');
Yes, you can. By using the CustomEvent API for the browser environment. And using an event emitter project in node.js environments. Since the snippet in the question is for the browser environment, here is a working example for the same.
function myPromiseReturningFunction(){
return new Promise(resolve => {
window.addEventListener("myCustomEvent", (event) => {
resolve(event.detail);
})
})
}
myPromiseReturningFunction().then(result => {
alert(result)
})
document.getElementById("p").addEventListener("click", () => {
window.dispatchEvent(new CustomEvent("myCustomEvent", {detail : "It works!"}))
})
<p id="p"> Click me </p>
I hope this answer is useful!
Thanks to everyone who posted in this thread. I created a module that includes the Defer() object described earlier as well as a few other objects built upon it. They all leverage Promises and the neat Promise call-back syntax to implement communication/event handling within a program.
Defer: Promise that can be resolved failed remotely (outside of its body)
Delay: Promise that is resolved automatically after a given time
TimeOut: Promise that fails automatically after a given time.
Cycle: Re-triggerable promise to manage events with the Promise syntax
Queue: Execution queue based on Promise chaining.
rp = require("openpromise")
https://github.com/CABrouwers/openpromise
https://www.npmjs.com/package/openpromise
Class version, in Typescript :
export class Deferred<T> {
public readonly promise: Promise<T>
private resolveFn!: (value: T | PromiseLike<T>) => void
private rejectFn!: (reason?: any) => void
public constructor() {
this.promise = new Promise<T>((resolve, reject) => {
this.resolveFn = resolve
this.rejectFn = reject
})
}
public reject(reason?: any): void {
this.rejectFn(reason)
}
public resolve(param: T): void {
this.resolveFn(param)
}
}
I wrote a small lib for this. https://www.npmjs.com/package/#inf3rno/promise.exposed
I used the factory method approach others wrote, but I overrode the then, catch, finally methods too, so you can resolve the original promise by those as well.
Resolving Promise without executor from outside:
const promise = Promise.exposed().then(console.log);
promise.resolve("This should show up in the console.");
Racing with the executor's setTimeout from outside:
const promise = Promise.exposed(function (resolve, reject){
setTimeout(function (){
resolve("I almost fell asleep.")
}, 100000);
}).then(console.log);
setTimeout(function (){
promise.resolve("I don't want to wait that much.");
}, 100);
There is a no-conflict mode if you don't want to pollute the global namespace:
const createExposedPromise = require("#inf3rno/promise.exposed/noConflict");
const promise = createExposedPromise().then(console.log);
promise.resolve("This should show up in the console.");
I made a library called manual-promise that functions as a drop in replacement for Promise. None of the other answers here will work as drop in replacements for Promise, as they use proxies or wrappers.
yarn add manual-promise
npn install manual-promise
import { ManualPromise } from "manual-promise";
const prom = new ManualPromise();
prom.resolve(2);
// actions can still be run inside the promise
const prom2 = new ManualPromise((resolve, reject) => {
// ... code
});
new ManualPromise() instanceof Promise === true
https://github.com/zpxp/manual-promise#readme
Just another solution to resolve Promise from the outside
class Lock {
#lock; // Promise to be resolved (on release)
release; // Release lock
id; // Id of lock
constructor(id) {
this.id = id
this.#lock = new Promise((resolve) => {
this.release = () => {
if (resolve) {
resolve()
} else {
Promise.resolve()
}
}
})
}
get() { return this.#lock }
}
Usage
let lock = new Lock(... some id ...);
...
lock.get().then(()=>{console.log('resolved/released')})
lock.release() // Excpected 'resolved/released'
How about creating a function to hijack the reject and return it ?
function createRejectablePromise(handler) {
let _reject;
const promise = new Promise((resolve, reject) => {
_reject = reject;
handler(resolve, reject);
})
promise.reject = _reject;
return promise;
}
// Usage
const { reject } = createRejectablePromise((resolve) => {
setTimeout(() => {
console.log('resolved')
resolve();
}, 2000)
});
reject();
I've put together a gist that does that job: https://gist.github.com/thiagoh/c24310b562d50a14f3e7602a82b4ef13
here's how you should use it:
import ExternalizedPromiseCreator from '../externalized-promise';
describe('ExternalizedPromise', () => {
let fn: jest.Mock;
let deferredFn: jest.Mock;
let neverCalledFn: jest.Mock;
beforeEach(() => {
fn = jest.fn();
deferredFn = jest.fn();
neverCalledFn = jest.fn();
});
it('resolve should resolve the promise', done => {
const externalizedPromise = ExternalizedPromiseCreator.create(() => fn());
externalizedPromise
.promise
.then(() => deferredFn())
.catch(() => neverCalledFn())
.then(() => {
expect(deferredFn).toHaveBeenCalled();
expect(neverCalledFn).not.toHaveBeenCalled();
done();
});
expect(fn).toHaveBeenCalled();
expect(neverCalledFn).not.toHaveBeenCalled();
expect(deferredFn).not.toHaveBeenCalled();
externalizedPromise.resolve();
});
...
});
As I didn't find what I was looking for, I will share what I actually wanted to achieve when I ended in this question.
Scenario: I have 3 different API's with same possible response and therefore I would like to handle the completion and error handling of the promises in a single function. This is what I did:
Create a handler function:
private handleHttpPromise = (promise: Promise<any>) => {
promise
.then((response: any) => {
// do something with the response
console.log(response);
})
.catch((error) => {
// do something with the error
console.log(error);
});
};
Send your promises to the created handler
switch (method) {
case 'get': {
this.handleHttpPromise(apiService.get(url));
break;
}
case 'post': {
if (jsonData) {
this.handleHttpPromise(apiService.post(url, jsonData));
}
break;
}
// (...)
}
I would like to share something different, an extension to this topic.
Sometimes you want a "task promise" to be automatically re-created at the same address (property or variable) when it resolves. It's possible to create an outside resolver that does just that.
Example of a recurring promise with an external resolver. Whenever the resolver is called, a new promise is created at the same address/variable/property.
let resolvePromise;
let thePromise;
const setPromise = (resolve) => {
resolvePromise = () => {
resolve();
thePromise = new Promise(setPromise);
}
}
thePromise = new Promise(setPromise);
(async () => {
let i = 0;
while (true) {
let msg = (i % 2 === 0) ? 'Tick' : 'Tock';
document.body.innerHTML = msg;
setTimeout(resolvePromise, 1000);
await thePromise;
i++;
}
})();
https://jsfiddle.net/h3zvw5xr
If (like me) you don't like augmenting native instances, nor unwieldy ".promise" properties ... but do love proxies and mangling classes, then this one is for you:
class GroovyPromise {
constructor() {
return new Proxy(new Promise((resolve, reject) => {
this.resolve = resolve;
this.reject = reject;
}), {
get: (target, prop) =>
this[prop] || target[prop].bind(target),
});
}
}
Used like so:
const groovypromise = new GroovyPromise();
setTimeout(() => groovypromise.resolve('groovy'), 1000);
console.log(await groovypromise);
Of course you can also rename the class to something dull like "Deferred"
For fun, you also combine a promise into a self-resolvable function:
function Resolver() {
let resolve;
const promise = new Promise(r => resolve = r);
return new Proxy(resolve, {
get: (_, prop) => promise[prop].bind(promise)
});
}
const resolve = Resolver();
(async () => {
resolve
.then(value => console.log('thenable:', value))
.finally(() => console.log('finally'));
const value = await resolve;
console.log('awaitable:', value);
})()
resolve('test');
// thenable: test
// finally
// awaitable: test
first enable --allow-natives-syntax on browser or node
const p = new Promise(function(resolve, reject){
if (someCondition){
resolve();
} else {
reject();
}
});
onClick = function () {
%ResolvePromise(p, value)
}