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JS Promises: is there an optional way to resolve multiple promises as properties of an object?

I wrote the code below and then realized what it resolved before (registration before all promises resolve):

readDirPromise
.then(categoriseFiles)
.then(({movies, series}) => ({
  movies: Promise.all(movies.map(movieTasks)),
  series: Promise.all(series.map(seriesTasks))
}))
.then((res) => {
  console.log('🦄 done!', res)
})

I was able to rewrite it to resolve it in the correct order:

readDirPromise
.then(categoriseFiles)
.then((cats) => Promise.all(cats.movies.map(movieTasks)).then((movies) => {
  cats.movies = movies
  return cats
}))
.then((cats) => Promise.all(cats.series.map(seriesTasks)).then((series) => {
  cats.series = series
  return cats
}))
.then((res) => {
  console.log('🦄 done!', res)
})

but I can't help but think ... is there a neater, more extensible way?

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3 answers


You can make the serial step independent of the video step (rather than block one by one) by wrapping both of them on a different layer Promise.all

, returning as a tuple, which you can then destruct and rebuild into the object you want:



readDirPromise
.then(categoriseFiles)
.then(({movies, series}) => Promise.all([
  Promise.all(movies.map(movieTasks)),
  Promise.all(series.map(seriesTasks))])
.then(([movies, series]) => ({movies, series}))
.then((res) => {
  console.log('🦄 done!', res)
})
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For what it's worth, I believe the async / await version would look something like this:



async function generateResponse() {
  const {movies, series} = await readDirPromise.then(categoriseFiles);
  return {
    movies: await Promise.all(movies.map(movieTasks)),
    series: await Promise.all(series.map(seriesTasks))
  }
}
+2


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Instead of chaining promises, you can create separate promises and resolve them in the method Promise.all

as you did.

var p1 = getNewPromise();
var p2 = getNewPromise();
var p3 = getNewPromise();

// From MDN

Promise.all([p1, p2, p3]).then(values => { 
  console.log(values); 
});
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