How to get the index of the Promise returned by Promise.any from an array of Promises?

Question

I'm trying to implement a queue of promises that fetch images. So, I create an array of 3 promises and get the first fetched image using Promise.any() . But when I receive the first image, I want to replace the used promise with a new one. Example:

const images = [
    fetch("some/image"),
    fetch("another/image"),
    fetch("another/image"),
];

Promise.any(images).then(img => {
    images[n] = fetch("another/image") // if only I knew "n"
    return img
}).then(/*whatever*/)

I've tried finding the index by comparing each promise of the array to the one returned by Promise.any , but that didn't work since it returns a new promise.

So, how do I know the index of the promise who's value was returned by Promise.any 's promise?

Answer 1

You can add index for each image promise before passing them to Promise.any (add .then chain)

 const images = [ Promise.resolve("some/image1"), Promise.resolve("some/image2"), Promise.resolve("some/image3"), ]; Promise.any(images.map((p, i) => p.then(v => [v, i]))).then(([img, index]) => { console.log('image', img) console.log('index', index) })

Answer 2

Consider using promise-limit package for this purpose. It allows you to solve your problem in a more robust way:

var promiseLimit = require('promise-limit')
 
var limit = promiseLimit(2) // max count of simultaneous requests
 
var allImages = ['a', 'b', 'c', 'd', 'e']
 
Promise.all(allImages.map((name) => {
  return limit(() => fetch(name).then(imageLoadedCallback))
}))