#promise.all

[codewithnazam]

Mastering Asynchronous with JavaScript Promise allSettled()

Introduction to JavaScript Promises What are Promises? When to Use Promises? Benefits of Promises Creating Promises with the Promise Constructor Understanding the allSettled() Method Functionality of allSettled() Differences from Promise.all() When to Use allSettled() Benefits of Using allSettled() Handling Multiple Asynchronous Operations Simultaneously Dealing with Mixed Fulfilled and…

[transienturl]

Hrmmmm.

I have a piece of code that (simplified) takes an array of values, runs a fetch corresponding to each value, and does something with the results (in order). The naive way to do this runs all of the fetches in sequence, which is of course slow.

for (const value of someValues) { const result = await fetch(value); console.log(result); }

The slightly less naive way to do this (Promise.all) fires all of the fetches at the same time, which is fine if you have a small number, but not so much if you have a big number. Ideally you would want to do a specific configurable number of fetches at once.

Obviously this isn't hard to do if you write your code that way in advance. I was trying to think of a way to do a helper function that could be used to replace the naive way with a minimal change to the code, though.

In a way, I actually already had one of these (this gist needs more comments; the technique is quite cool imho but it's a bit of a fun puzzle to understand):

This is nice for medium-sized arrays where you're okay with waiting until all of the fetches to resolve before you do anything else with them. But for really large arrays, it's nice to be able to do stuff with the results incrementally. I had this idea:

const batchCallAsync = (inputValues, cb, batchSize) => { const results = []; const values = [...inputValues]; let waitForPreviousBatch = Promise.resolve(); while (values.length) { const resultBatch = values.splice(0, batchSize).map( (value) => new Promise((resolve) => { waitForPreviousBatch.finally(() => resolve(cb(value))); }) ); waitForPreviousBatch = Promise.allSettled(resultBatch); results.push(...resultBatch); } return results; };

This, when run, instantly generates an array of promises, but the promises execute fetches and resolve in batches. You can thus do e.g.

for await (const result of batchCallAsync(someValues, fetch, 5)) { console.log(result) }

and the log statements will come out in real time as the fetches occur. Neat. Could use some refactoring (and a performance improvement to match that gist), but whatever.

Problem, though: you can't cancel this. If you call this on an array of length 3500, you are getting 3500 fetches executed even if you break that for loop (unless you close node/your browser tab). That's not true of the original snippet.

I'm pretty sure it's possible to solve that, too? This seems like the ideal use case for an async generator function. Haven't bothered to write it, yet, but it should be quite simple in principle.

[hellowahab]

30 Must-Know Async Problems for JavaScript Interviews

1️⃣ Build a custom Promise from scratch.2️⃣ Create your own Promise.all implementation.3️⃣ Design a Promise.any that resolves to the first fulfilled promise.4️⃣ Develop a Promise.race to resolve based on the fastest result.5️⃣ Implement Promise.allSettled to handle multiple results—fulfilled or rejected.6️⃣ Add a finally method for promises that always runs, regardless of outcome.7️⃣ Convert…

[hautevaux]

@draikoeques asked:

A small velvet box is left upon his nightstand, a carefully scrawled note beside it. The handwriting isn't pretty by any means but it is clear and le gible, obviously done with slow care likely early in the morning. The note read, Good morning, dearest, and happy birthday. I wished to give you a small gift for the occasion, but have a few errands to run and didn't want to miss you. I will be back soon, I promise.All my love,Rothalion. Within the box was a necklace. The chain and pendant were rose gold, a diamond and two pearls nestled in the center. It would be but a few minutes following Vaux's awakening that the sound of a door could be heard, Rothalion finally returning with another larger box in his hands and an impossible to hide smile upon his face. He had managed to purchase a cake as well, although some of his time away was spent trying to decide exactly which one to buy! As a man who has never once celebrated his own birthday he feels it all the more important to celebrate that of someone he cared the most for.

Long nights and early starts were a strain on the tailor of whom was struggling to wake up that morn. The sun had only just begun to rise, and Vaux had groaned and hidden his head under a pillow for a few moments ere he reluctantly sat himself up. Pale hair was quickly pulled over one shoulder - body aches and pains briefly noted ere he spins legs around to sit 'pon the edge of his bed.

Only then, after idly rubbing his face to try and rid it of fatigue, does he note the box and note left aside his bed and with curiosity does he reach for it. Confused, Vaux unfolds the note and reads every word with yet groggy eyes; Birthday-? A moment of thought, a realisation.

He's neither remembered nor considered his birthday in a long time, and a strange variety of conflict nests within his chest for a moment ere he turns to the box - evidently a gift.

With deft fingers does he open it, smiling quietly at the contents, the tip of a finger brushing gingerly against the stones resting within. He's barely had a moment to glance upon it ere he heard the door to his home open, and Ro appearing in the doorway.

Vaux's expression is still one of surprise, of sentiment, of conflicting emotions stemming from poor treatment throughout childhood; but he smiles, a moment of genuine joy striking.

"Is this truly for me-?"

[ishiduca]

isaacsのglobをfs.globで置き換えるみたいなのを始めたけど、だいたい配列のmapをPromise.allで処理してたので、Array.fromAsyncにAsyncIteratorとmapFnを渡せば良いだけということがわか(るまで大変だ)った (5月16日9時8分)

[weva-cloud]

特定ページ上の特定クラスのリンク先URLを収集し、 フロントJSでそれら���HTMLを開いて、 それぞれのページ上でさらに特定クラスのリンク先URLを集めるためのスクリプト例：

```js

var links = Array.from(document.querySelectorAll('a.target-class')).map(a => a.href);

var promiseResponseTexts = await Promise.all( links.map(link => fetch(link)) ) .then(res => res.map(r => r.text()));

var responseTexts = await Promise.all(promiseResponseTexts);

var nextPageTargetLinks = responseTexts.map(text => { var parser = new DOMParser(); var doc = parser.parseFromString(text, 'text/html'); return doc.querySelector('a.next-page-target-class')?.href; });

console.log(nextPageTargetLinks)

```

これを応用するとフロントエンドJavaScriptだけで柔軟なスクレイピングが可能です。

[james-clarksons]

JavaScript Promises Explained: Asynchronous Programming Made Easy

Asynchronous programming in JavaScript can be a challenging concept to grasp, but it's a crucial skill for any web developer. It allows you to execute tasks without blocking the main thread, ensuring a smooth and responsive user experience. One of the key tools for handling asynchronous operations in JavaScript is Promises. In this blog post, we'll explore what Promises are, why they're important, and how they make asynchronous programming more manageable.

Understanding Asynchronous Programming

In JavaScript, many operations take time to complete, such as fetching data from an API, reading a file, or waiting for user input. In a traditional synchronous program, these operations would block the entire application until they're finished, leading to unresponsive and sluggish user interfaces. Asynchronous programming solves this problem by allowing tasks to run in the background while the main program continues to execute.

Introducing Promises

A Promise in JavaScript represents a value that may not be available yet but will be resolved at some point in the future. It provides a way to work with asynchronous operations in a more structured and readable manner.

Promises have three states:

Pending: The initial state when the Promise is created and hasn't been resolved or rejected.

Fulfilled: The state when the asynchronous operation is successful, and the Promise has a result.

Rejected: The state when the asynchronous operation encounters an error, and the Promise has a reason for the failure.

Here's a basic example of a Promise:const myPromise = new Promise((resolve, reject) => { // Simulate an asynchronous operation setTimeout(() => { const success = true; if (success) { resolve("Operation was successful"); } else { reject("Operation failed"); } }, 2000); // Resolves or rejects after 2 seconds }); // Using the Promise myPromise .then((result) => { console.log(result); // Operation was successful }) .catch((error) => { console.error(error); // Operation failed });

In this example, the Promise represents an operation that either succeeds (fulfills) or fails (rejects) after a delay of 2 seconds. You can use the .then() method to handle the fulfilled state and the .catch() method to handle the rejected state.

Advantages of Promises

Promises offer several advantages for managing asynchronous code:

1. Improved Readability

Promises provide a clear and structured way to handle asynchronous operations, making your code more readable and maintainable. This is especially important when dealing with complex workflows that involve multiple asynchronous tasks.

2. Error Handling

Promises make it easy to handle errors in asynchronous code. You can use the .catch() method to catch and handle errors from any part of the Promise chain, enhancing error management and debugging.

3. Chaining

Promises can be chained together, allowing you to sequence asynchronous operations. This avoids the "callback hell" problem, where nested callbacks can become difficult to manage.

4. Support for Parallel Execution

You can use Promise.all() to execute multiple asynchronous operations in parallel and wait for all of them to complete. This is useful for scenarios where you need to fetch data from multiple sources simultaneously.

Async/Await: Syntactic Sugar for Promises

While Promises are a powerful tool for managing asynchronous code, they can sometimes lead to nested code blocks, especially when chaining multiple Promises. To address this, JavaScript introduced async/await syntax, which provides a more synchronous-looking way to work with Promises.

Here's an example using async/await:async function fetchData() { try { const response = await fetch("https://api.example.com/data"); const data = await response.json(); console.log(data); } catch (error) { console.error(error); } } fetchData();

async/await simplifies the syntax and makes asynchronous code more readable by allowing you to write asynchronous code in a way that resembles synchronous code.

JavaScript Promises are a fundamental part of modern web development, enabling you to work with asynchronous operations in a structured and efficient manner. By understanding how Promises work and how to use them effectively, you can build responsive and user-friendly web applications.

Asynchronous programming is a key skill for web developers, and Promises are an essential tool in your toolkit. Whether you're fetching data from an API, handling user interactions, or performing any other asynchronous task, Promises simplify the process and make your code more robust and maintainable.

So, the next time you encounter asynchronous code in your JavaScript projects, embrace Promises and async/await to streamline your development workflow and create more responsive web applications.

[techwebdevelopment]

JavaScript Promises Unleashed: The Epic Battle of any vs race vs all!

In short:Promise.all: Returns all resolved values or first rejected. Promise.any: Returns first resolved or aggregated all errors Promise.race: Returns first resolved or rejected Example1: const promise1 = Promise.resolve(“Hello”); const promise2 = Promise.resolve(“World”); const promise3 = Promise.resolve(“!”); Promise.all([promise1, promise2, promise3]) .then((results) => console.log(“Success:” + results)) .catch((error) => console.log(“Error:” + error)); > Success:Hello,World,! Promise.any([promise1, promise2, promise3]) .then((results) […] The post JavaScript Promises Unleashed: The Epic Battle of any vs race vs all! appeared first on TECH - WEB DEVELOPMENT NEWS. https://tech-webdevelopment.news-6.com/javascript-promises-unleashed-the-epic-battle-of-any-vs-race-vs-all/

[obscurejavascript]

Waiting For All Promises to Execute in JavaScript (Even if There Are Errors)

Promise.all takes an array of promises to run all at once and stops either when all promises are successful or when one promise is rejected. In a lot of cases this is what is wanted. Though sometimes all promises need to be waited for no matter what and the operations should happen in parallel. For example, to track all errors found so a unified error message can be generated. Here is what happens when using Promise.all:

// Simulated API Calls function api1() { return new Promise((resolve, reject) => { setTimeout(() => reject('Error 1'), 200); }); } function api2() { return new Promise((resolve, reject) => { setTimeout(() => resolve(['some', 'data']), 300); }); } function api3() { return new Promise((resolve, reject) => { setTimeout(() => reject('Error 2'), 400); }); } async function displayAllResults() { try { const results = await Promise.all([api1(), api2(), api3()]); console.log('results', results); // (never called) } catch (err) { console.log(err); // Error 1 } } displayAllResults();

Instead a function that only is fulfilled when all promises have run (Promise.allSettled) no matter what:

async function displayAllResults() { try { const results = await Promise.allSettled([api1(), api2(), api3()]); const errors = results.filter((result) => result.status === 'rejected'); console.log(errors.map((error) => error.reason)); // [ 'Error 1', 'Error 2' ] } catch (err) { console.info(err); // (never called) } } displayAllResults();

This function will soon be available in native JavaScript: https://github.com/tc39/proposal-promise-allSettled. Here is my implementation to demonstrate the above:

if (typeof Promise.allSettled !== 'function') { const alwaysResolveWrapper = function(promise) { return promise.then((value) => { return { status: 'fulfilled', value }; }, (err) => { return { status: 'rejected', reason: err }; }); } Promise.allSettled = function(promises) { return Promise.all(promises.map((promise) => { return alwaysResolveWrapper(promise); })); } }

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2019/allSettled.js

[iamprogrammerz]

JavaSript Promises: Promise.all vs Promise.race ☞ https://morioh.com/p/03f32017823a #JavaScript #Morioh

[codewithnazam]

Mastering Asynchronous with JavaScript Promise allSettled()

Harness the power of Promise.allSettled() to handle multiple asynchronous operations with ease in JavaScript. This comprehensive guide teaches you everything you need to know.

Introduction to JavaScript Promises What are Promises? When to Use Promises? Benefits of Promises Creating Promises with the Promise Constructor Understanding the allSettled() Method Functionality of allSettled() Differences from Promise.all() When to Use allSettled() Benefits of Using allSettled() Handling Multiple Asynchronous Operations Simultaneously Dealing with Mixed Fulfilled and…

[transienturl]

const delayPromises = []; export const throttledApiFetch = (...args) => new Promise((resolve) => { const currentPromises = [...delayPromises]; delayPromises.push( // eslint-disable-next-line promise/param-names new Promise((resolveDelay) => Promise.all(currentPromises).then(() => { resolve(apiFetch(...args)); setTimeout(resolveDelay, 1000); }) ) ); });

now this is cursed. effectively unbounded memory usage, nested promise constructors.

[alljavascript]

Promise.all using async await

Promise.all = async function (promises) {   let results = [];   let errorIndex = -1;   for (let i = 0; i < promises.length; i++) {     try {       results[i] = await promises[i];     } catch (e) {       if (errorIndex === -1) {         results[i] = e;         errorIndex = i;       }     }   }   if (errorIndex === -1) {     return results;   } else {     throw results[errorIndex];   } }

[themailhopz-blog]

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[canmom]

the experience of programming

i need to load assets to my game. i’ll use the fetch API because async is sexy

i need to wait to start the game until all the promises have been loaded. I’ll store the state of each one in an object like {script: false, images: false}, and write a function which checks all the relevant fields of the object to see if they’re all true with a long string of &&s, and then on each of the promises I’ll call .then() so that when they resolve, I can set that one’s entry to true and call the checking function... nice

wait a minute, promises are already objects and I can just look up their state. I’ll just store the promises in an array, and map a function that calls .then() with a function to check on the state of all the promises over that array. that "check the other promises” function can .reduce() the array with a function that calls && on each pair. (hehe, I’m a functional programmer now! that way, I can add as many new asset-loading promises as I like with no trouble!)

wait a minute, that’s going to involve a lot of redundant checks instead of stopping as soon as it finds one that’s false. I wonder what the standard way to check if an array is all true... oh there’s a .every() function that probably short-circuits, right? regardless that will save all that map-reduce faff

now I just need to look up the promise API to find out how to know if a promise has been resolved and--

oh for fuck’s sake, Promise.all() exists, and I’ve just been clunkily reinventing a wheel that’s been right there all along :D