promise.all inside a forEach loop — everything firing at once

Question

In a Node app, I need to iterate through some items in a synchronous fashion, but some of the operations inside the loop are asynchronous. My code right now looks like so:

someAPIpromise().then((items) => {
   items.forEach((item) => {
      Promise.all[myPromiseA(item), myPromiseB(item)]).then(() => {
         doSomethingSynchronouslyThatTakesAWhile();
      });
    }
}

This works wonders when the items is an array of 1. But, once there's more than one item, promise.all() will just fire off instantly for every item in the array, without waiting for the operation in the loop to end.

All that to say... how can I ensure that the entire operation for each item in the array is run synchronously (even if some operations are async and return a promise)?

Thanks so much!

N

Answer 1

You're constructing several promises, but they are all asynchronous. You construct Promise1, Promise2, Promise3, ... but once they're in the wild they are all firing simultaneously. If you want synchronous behavior you've got to chain them together so Promise1's .then() executes Promise2 and so on. In the past I've used Array.reduce for this.

someAPIpromise().then((items) => {
    items.reduce((accumulator, current) =>
        accumulator.then(() =>
             Promise.all[myPromiseA(item), myPromiseB(item)]).then(() => 
                 doSomethingSynchronouslyThatTakesAWhile();
             )
        )
    , Promise.resolve());

You can write this as a helper function if you like, which may make things clearer.

function execSequentially (arr, func) {
    return arr.reduce(
        (accumulator, current) => accumulator.then(() => func(current)), 
        Promise.resolve());
}

That function is executed as

execSequentially(items, item => console.log(item));

of course replacing console.log with what you want to do.

The helper function approach is also less invasive of a change. The helper applied to your original code:

someAPIpromise().then((items) => {
   execSequentially(items, (item) =>
      Promise.all[myPromiseA(item), myPromiseB(item)]).then(() => {
         doSomethingSynchronouslyThatTakesAWhile();
      });
   );
});