How to return many Promises and wait for them all before doing other stuff

Question

I have a loop which calls a method that does stuff asynchronously. This loop can call the method many times. After this loop, I have another loop that needs to be executed only when all the asynchronous stuff is done.

So this illustrates what I want:

for (i = 0; i < 5; i++) {
    doSomeAsyncStuff();    
}

for (i = 0; i < 5; i++) {
    doSomeStuffOnlyWhenTheAsyncStuffIsFinish();    
}

I'm not very familiar with promises, so could anyone help me to achieve this?

This is how my doSomeAsyncStuff() behaves:

function doSomeAsyncStuff() {
    var editor = generateCKEditor();
    editor.on('instanceReady', function(evt) {
        doSomeStuff();
        // There should be the resolve() of the promises I think.
    })
}

Maybe I have to do something like this:

function doSomeAsyncStuff() {
    var editor = generateCKEditor();
    return new Promise(function(resolve,refuse) {
        editor.on('instanceReady', function(evt) {
            doSomeStuff();
            resolve(true);
        });
    });
}

But I'm not sure of the syntax.

Answer 1

You can use Promise.all (spec, MDN) for that: It accepts a bunch of individual promises and gives you back a single promise that is fulfilled when all of the ones you gave it are fulfilled, or rejected when any of them is rejected. The results in the fulfillment array are in the same order as the promises in the input array.

So if you make doSomeAsyncStuff return a promise, then:

    const promises = [];
//  ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−− use `const` or `let`, not `var`
    
    for (let i = 0; i < 5; i++) {
//       ^^^−−−−−−−−−−−−−−−−−−−−−−−− added missing declaration
        promises.push(doSomeAsyncStuff());
    }
    
    Promise.all(promises)
        .then(() => {
            for (let i = 0; i < 5; i++) {
//               ^^^−−−−−−−−−−−−−−−− added missing declaration
                doSomeStuffOnlyWhenTheAsyncStuffIsFinish();    
            }
        })
        .catch((e) => {
            // handle errors here
        });

MDN has an article on promises here. I also cover promsies in detail in Chapter 8 of my book JavaScript: The New Toys, links in my profile if you're interested.

Here's an example:

function doSomethingAsync(value) {
    return new Promise((resolve) => {
        setTimeout(() => {
            console.log("Resolving " + value);
            resolve(value);
        }, Math.floor(Math.random() * 1000));
    });
}
   
function test() {
    const promises = [];
       
    for (let i = 0; i < 5; ++i) {
        promises.push(doSomethingAsync(i));
    }
       
    Promise.all(promises)
        .then((results) => {
            console.log("All done", results);
        })
        .catch((e) => {
            // Handle errors here
        });
}
   
test();

Sample output (because of the Math.random, what finishes first may vary, but the "All done" line will be at the end):

Resolving 3
Resolving 2
Resolving 1
Resolving 4
Resolving 0
All done [0,1,2,3,4]

Note, again, that the contents of the fulfillment array are in the order of the promises in the input array (not the order the promises were fulfilled in).

Answer 2

Here's an elegant solution for you if you want to do the same thing multiple times:

await Promise.all(new Array(10).fill(0).map(() => asyncFn()));

This creates an array with 10 items, fills it with zeros and then maps it to an array of promises.

Answer 3

/*** Worst way ***/
for(i=0;i<10000;i++){
  let data = await axios.get(
    "https://yourwebsite.com/get_my_data/"
  )
  //do the statements and operations
  //that are dependant on data
}

//Your final statements and operations
//That will be performed when the loop ends

//=> this approach will perform very slow as all the api call
// will happen in series


/*** One of the Best way ***/

const yourAsyncFunction = async (anyParams) => {
  let data = await axios.get(
    "https://yourwebsite.com/get_my_data/"
  )
  //all you statements and operations here
  //that are dependant on data
}
var promises = []
for(i=0;i<10000;i++){
  promises.push(yourAsyncFunction(i))
}
await Promise.all(promises)
//Your final statement / operations
//that will run once the loop ends

//=> this approach will perform very fast as all the api call
// will happen in parallal

Answer 4

Here is code that I wrote for myself in order to understand the answers stated here. I have mongoose queries in a for loop, so I put here the asyncFunction to take its place. Hope it helps anyone. You can run this script in node or any of many Javascript runtimes.

let asyncFunction = function(value, callback)
{
        setTimeout(function(){console.log(value); callback();}, 1000);
}



// a sample function run without promises

asyncFunction(10,
    function()
    {
        console.log("I'm back 10");
    }
);


//here we use promises

let promisesArray = [];

let p = new Promise(function(resolve)
{
    asyncFunction(20,
        function()
        {
            console.log("I'm back 20");
            resolve(20);
        }
    );
});

promisesArray.push(p);


for(let i = 30; i < 80; i += 10)
{
    let p = new Promise(function(resolve)
    {
        asyncFunction(i,
            function()
            {
                console.log("I'm back " + i);
                resolve(i);
            }
        );
    });
    promisesArray.push(p);
}


// We use Promise.all to execute code after all promises are done.

Promise.all(promisesArray).then(
    function()
    {
        console.log("all promises resolved!");
    }
)

Answer 5

const doSomeAsyncStuff = async (funcs) => {
  const allPromises = funcs.map(func => func());
  return await Promise.all(allPromises);
}

doSomeAsyncStuff([
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(), 100)),
]);

Answer 6

A reusable function works nicely for this pattern:

function awaitAll(count, asyncFn) {
  const promises = [];

  for (i = 0; i < count; ++i) {
    promises.push(asyncFn());
  }

  return Promise.all(promises);
}

OP example:

awaitAll(5, doSomeAsyncStuff)
  .then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
  .catch(e => console.error(e));

---

A related pattern, is iterating over an array and performing an async operation on each item:

function awaitAll(list, asyncFn) {
  const promises = [];

  list.forEach(x => {
    promises.push(asyncFn(x));
  });

  return Promise.all(promises);
}

Example:

const books = [{ id: 1, name: 'foo' }, { id: 2, name: 'bar' }];

function doSomeAsyncStuffWith(book) {
  return Promise.resolve(book.name);
}

awaitAll(books, doSomeAsyncStuffWith)
  .then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
  .catch(e => console.error(e));