一系列承诺会在完成后立即解决承诺吗？

Question

I have an array of Promises, and I want to trigger certain actions on them as soon as they complete. Promises.all isn't quite what I'm looking for since it waits until all Promises in the Iterable have completed. Promises.race returns whatever the first Promise which complete returns.

Assume that you can only use: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

Answer 1

Taking into account you want to use Vanilla JS, if you want they to execute them concurrently and as soon as they are resolved you could do something like this:

// create a Promise that is resolved after passed millisecs with millisecs * 2 as value
const createP = (ms) => new Promise(resolve => setTimeout(() => resolve(ms * 2), ms));

// your array of Promises
const pArray = [createP(1000), createP(200), createP(500)];

// map() and Promise.all() is necessary in order to wait until all the promises are executed
Promise.all(pArray.map(promise => { 
  // log each result
  return promise.then(console.log);
}))
.catch(err =>{
  // do some error handling here if necessary
});

// it should log 400, 1000, 2000 in this order

Answer 2

You can create a method promiseSerial that will resolve promises sequentially instead of in parallel.

Here's an example implementation:

/*
* promiseSerial resolves Promises sequentially.
* @example
* const urls = ['/url1', '/url2', '/url3']
* const funcs = urls.map(url => () => $.ajax(url))
*
* promiseSerial(funcs)
*   .then(console.log)
*   .catch(console.error)
*/
const promiseSerial = funcs =>
  funcs.reduce((promise, func) =>
    promise.then(result => func().then(Array.prototype.concat.bind(result))),
    Promise.resolve([]))

// some url's to resolve
const urls = ['/url1', '/url2', '/url3']

// convert each url to a function that returns a promise
const funcs = urls.map(url => () => $.ajax(url))

// execute Promises in serial
promiseSerial(funcs)
  .then(console.log)
  .catch(console.error)

from: https://hackernoon.com/functional-javascript-resolving-promises-sequentially-7aac18c4431e