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data/en/blog/does-promise-all-run-in-parallel-or-sequential.mdx
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title: 'Does JavaScript Promise.all() run in parallel or sequential?' | ||
date: '2023-09-04' | ||
tags: ['javascript', 'promise', 'parallel', 'sequential', 'concurrently'] | ||
draft: false | ||
summary: 'Does JavaScript Promise.all run in parallel or sequential?' | ||
images: ['/static/images/road.jpg'] | ||
authors: ['default'] | ||
--- | ||
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import Twemoji from './Twemoji.tsx' | ||
import UnsplashPhotoInfo from './UnsplashPhotoInfo.tsx' | ||
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![thumbnail-image](/static/images/road.jpg) | ||
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<UnsplashPhotoInfo photoURL="https://unsplash.com/photos/rafblRbne3o" author="Karsten Würth" /> | ||
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Let's say you have a list of async tasks (each return a **Promise**). | ||
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```javascript | ||
let promise1 = async function () { | ||
/* ... */ | ||
} | ||
let promise2 = async function () { | ||
/* ... */ | ||
} | ||
let promise3 = async function () { | ||
/* ... */ | ||
} | ||
``` | ||
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What would you choose to run them? | ||
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Awaiting each promise one by one: | ||
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```javascript | ||
await promise1() | ||
await promise2() | ||
await promise3() | ||
// do other stuff | ||
``` | ||
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Or run them all at once: | ||
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```javascript | ||
await Promise.all([promise1(), promise2(), promise3()]) | ||
// do other stuff | ||
``` | ||
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The first approach is running them **sequentially**, one after another. It means that the next promise will start only after the previous one is resolved. | ||
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Like this: | ||
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```javascript | ||
promise1().then(() => { | ||
promise2().then(() => { | ||
promise3().then(() => { | ||
// do other stuff | ||
}) | ||
}) | ||
}) | ||
``` | ||
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The second approach is well-known as running them in **"parallel"**, meaning that all promises will start at the same time. | ||
It's useful when you don't need to wait for the previous promise to be resolved before starting the next one. | ||
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But does it really run in parallel (or all at once)? <Twemoji emoji="thinking-face" /> | ||
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The answer is no. JavaScript is single-threaded programming language, so it can't run multiple things at the exact same time (except for some circumstances such as [web workers](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers).) | ||
`Promise.all()` actually runs them concurrently, not in parallel! | ||
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What's the difference? | ||
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## Concurrent programming vs Parallel programming | ||
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> TL;DR: Concurrent programming is about dealing with a lot of things at once, while parallel programming is about doing a lot of things at once. | ||
See also this excellent explanation from [Haskell wiki](https://wiki.haskell.org/Parallelism_vs._Concurrency). | ||
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A dead-simple example for a 9-year-old kid: | ||
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- **Concurrency**: 2 lines of customers ordering food from a single cashier (lines take turns ordering). | ||
- **Parallelism**: 2 lines of customers ordering food at the same time from 2 cashiers. | ||
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As so, what `Promise.all()` does is, it adds the promises to an event loop queue and calls them all together. | ||
But it waits for each one to resolve before moving on. | ||
`Promise.all` will stop if the first promise rejects, unless you handle the error yourself (e.g. with `.catch()`). | ||
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That's the major difference between concurrent and parallel, with _concurrent execution_, promises run one after another but don't have to wait for previous ones to end. They make progress at the same time. | ||
In contrast, _parallel execution_ runs promises at the exact same time in separate processes. | ||
This allows them to progress completely separately at their own speed. | ||
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## Conclusion | ||
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The answer for the question in the title is: `Promise.all()` runs concurrently, all promises execute almost at the same time, but not in parallel. | ||
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If you have a list promises that don't depend on each other, you can run them concurrently (or parallel-like): | ||
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```javascript | ||
await Promise.all([promise1(), promise2(), promise3()]) | ||
// or | ||
await Promise.all( | ||
items.map(async (item) => { | ||
await doSomething(item) | ||
}) | ||
) | ||
``` | ||
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Or sequentially: | ||
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```javascript | ||
for (let item of items) { | ||
await doSomething(item) | ||
} | ||
``` | ||
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## References | ||
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- [`Promise.all()` documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all) | ||
- [Parallelism vs. Concurrency](https://wiki.haskell.org/Parallelism_vs._Concurrency) | ||
- [JavaScript Event Loop structure](https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop) | ||
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Happy promise-ing! <Twemoji emoji="clinking-beer-mugs" /> |
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