Node JS Promise.all and forEach

Here's a simple example using reduce. It runs serially, maintains insertion order, and does not require Bluebird.

/**
 * 
 * @param items An array of items.
 * @param fn A function that accepts an item from the array and returns a promise.
 * @returns {Promise}
 */
function forEachPromise(items, fn) {
    return items.reduce(function (promise, item) {
        return promise.then(function () {
            return fn(item);
        });
    }, Promise.resolve());
}

And use it like this:

var items = ['a', 'b', 'c'];

function logItem(item) {
    return new Promise((resolve, reject) => {
        process.nextTick(() => {
            console.log(item);
            resolve();
        })
    });
}

forEachPromise(items, logItem).then(() => {
    console.log('done');
});

We have found it useful to send an optional context into loop. The context is optional and shared by all iterations.

function forEachPromise(items, fn, context) {
    return items.reduce(function (promise, item) {
        return promise.then(function () {
            return fn(item, context);
        });
    }, Promise.resolve());
}

Your promise function would look like this:

function logItem(item, context) {
    return new Promise((resolve, reject) => {
        process.nextTick(() => {
            console.log(item);
            context.itemCount++;
            resolve();
        })
    });
}

It's pretty straightforward with some simple rules:

• Whenever you create a promise in a then, return it - any promise you don't return will not be waited for outside.
• Whenever you create multiple promises, .all them - that way it waits for all the promises and no error from any of them are silenced.
• Whenever you nest thens, you can typically return in the middle - then chains are usually at most 1 level deep.
• Whenever you perform IO, it should be with a promise - either it should be in a promise or it should use a promise to signal its completion.

And some tips:

• Mapping is better done with .map than with for/push - if you're mapping values with a function, map lets you concisely express the notion of applying actions one by one and aggregating the results.
• Concurrency is better than sequential execution if it's free - it's better to execute things concurrently and wait for them Promise.all than to execute things one after the other - each waiting before the next.

Ok, so let's get started:

var items = [1, 2, 3, 4, 5];
var fn = function asyncMultiplyBy2(v){ // sample async action
    return new Promise(resolve => setTimeout(() => resolve(v * 2), 100));
};
// map over forEach since it returns

var actions = items.map(fn); // run the function over all items

// we now have a promises array and we want to wait for it

var results = Promise.all(actions); // pass array of promises

results.then(data => // or just .then(console.log)
    console.log(data) // [2, 4, 6, 8, 10]
);

// we can nest this of course, as I said, `then` chains:

var res2 = Promise.all([1, 2, 3, 4, 5].map(fn)).then(
    data => Promise.all(data.map(fn))
).then(function(data){
    // the next `then` is executed after the promise has returned from the previous
    // `then` fulfilled, in this case it's an aggregate promise because of 
    // the `.all` 
    return Promise.all(data.map(fn));
}).then(function(data){
    // just for good measure
    return Promise.all(data.map(fn));
});

// now to get the results:

res2.then(function(data){
    console.log(data); // [16, 32, 48, 64, 80]
});

Just to add to the solution presented, in my case I wanted to fetch multiple data from Firebase for a list of products. Here is how I did it:

useEffect(() => {
  const fn = p => firebase.firestore().doc(`products/${p.id}`).get();
  const actions = data.occasion.products.map(fn);
  const results = Promise.all(actions);
  results.then(data => {
    const newProducts = [];
    data.forEach(p => {
      newProducts.push({ id: p.id, ...p.data() });
    });
    setProducts(newProducts);
  });
}, [data]);