Does JavaScript Promise.all() run in parallel or sequential?

Let's say you have a list of async tasks (each return a Promise).

let promise1 = async function () {
  /* ... */
}
let promise2 = async function () {
  /* ... */
}
let promise3 = async function () {
  /* ... */
}

What would you choose to run them?

Awaiting each promise one by one:

await promise1()
await promise2()
await promise3()
// do other stuff

Or run them all at once:

await Promise.all([promise1(), promise2(), promise3()])
// do other stuff

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.

Like this:

promise1().then(() => {
  promise2().then(() => {
    promise3().then(() => {
      // do other stuff
    })
  })
})

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.

But does it really run in parallel (or all at once)?

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.) Promise.all() actually runs them concurrently, not in parallel!

What's the difference?

Concurrent programming vs Parallel programming

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.

A dead-simple example for a 9-year-old kid:

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.

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()).

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.

Conclusion

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.

If you have a list promises that don't depend on each other, you can run them concurrently (or parallel-like):

await Promise.all([promise1(), promise2(), promise3()])
// or
await Promise.all(
  items.map(async (item) => {
    await doSomething(item)
  })
)

Or sequentially:

for (let item of items) {
  await doSomething(item)
}

References

Happy promise-ing!