How to synchronize a sequence of promises?

Question

Here are some simple examples for how you sequence through an array executing each async operation serially (one after the other).

Let’s suppose you have an array of items:

var arr = [...];

And, you want to carry out a specific async operation on each item in the array, one at a time serially such that the next operation does not start until the previous one has finished.

And, let’s suppose you have a promise returning function for processing one of the items in the array fn(item):

Manual Iteration

function processItem(item) {
    // do async operation and process the result
    // return a promise
}

Then, you can do something like this:

function processArray(array, fn) {
    var index = 0;
    
    function next() {
        if (index < array.length) {
            fn(array[index++]).then(next);
        }
    }
    next();
}

processArray(arr, processItem);

Manual Iteration Returning Promise

If you wanted a promise returned from processArray() so you’d know when it was done, you could add this to it:

function processArray(array, fn) {
    var index = 0;

    function next() {
        if (index < array.length) {
            return fn(array[index++]).then(function(value) {
                // apply some logic to value
                // you have three options here:
                // 1) Call next() to continue processing the result of the array
                // 2) throw err to stop processing and result in a rejected promise being returned
                // 3) return value to stop processing and result in a resolved promise being returned
                return next();
            });
        }
    } else {
        // return whatever you want to return when all processing is done
        // this returne value will be the ersolved value of the returned promise.
        return "all done";
    }
}

processArray(arr, processItem).then(function(result) {
    // all done here
    console.log(result);
}, function(err) {
    // rejection happened
    console.log(err);
});

Note: this will stop the chain on the first rejection and pass that reason back to the processArray returned promise.

Iteration with .reduce()

If you wanted to do more of the work with promises, you could chain all the promises:

function processArray(array, fn) {
   return array.reduce(function(p, item) {
       return p.then(function() {
          return fn(item);
       });
   }, Promise.resolve());
}

processArray(arr, processItem).then(function(result) {
    // all done here
}, function(reason) {
    // rejection happened
});

Note: this will stop the chain on the first rejection and pass that reason back to the promise returned from processArray().

For a success scenario, the promise returned from processArray() will be resolved with the last resolved value of your fn callback. If you wanted to accumulate a list of results and resolve with that, you could collect the results in a closure array from fn and continue to return that array each time so the final resolve would be an array of results.

Iteration with .reduce() that Resolves With Array

And, since it now seems apparent that you want the final promise result to be an array of data (in order), here’s a revision of the previous solution that produces that:

function processArray(array, fn) {
   var results = [];
   return array.reduce(function(p, item) {
       return p.then(function() {
           return fn(item).then(function(data) {
               results.push(data);
               return results;
           });
       });
   }, Promise.resolve());
}

processArray(arr, processItem).then(function(result) {
    // all done here
    // array of data here in result
}, function(reason) {
    // rejection happened
});

Working demo: http://jsfiddle.net/jfriend00/h3zaw8u8/

And a working demo that shows a rejection: http://jsfiddle.net/jfriend00/p0ffbpoc/

Iteration with .reduce() that Resolves With Array with delay

And, if you want to insert a small delay between operations:

function delay(t, v) {
    return new Promise(function(resolve) {
        setTimeout(resolve.bind(null, v), t);
    });
}

function processArrayWithDelay(array, t, fn) {
   var results = [];
   return array.reduce(function(p, item) {
       return p.then(function() {
           return fn(item).then(function(data) {
               results.push(data);
               return delay(t, results);
           });
       });
   }, Promise.resolve());
}

processArray(arr, 200, processItem).then(function(result) {
    // all done here
    // array of data here in result
}, function(reason) {
    // rejection happened
});

Iteration with Bluebird Promise Library

The Bluebird promise library has a lot of concurrency controlling features built right in. For example, to sequence iteration through an array, you can use Promise.mapSeries().

Promise.mapSeries(arr, function(item) {
    // process each individual item here, return a promise
    return processItem(item);
}).then(function(results) {
    // process final results here
}).catch(function(err) {
    // process array here
});

Or to insert a delay between iterations:

Promise.mapSeries(arr, function(item) {
    // process each individual item here, return a promise
    return processItem(item).delay(100);
}).then(function(results) {
    // process final results here
}).catch(function(err) {
    // process array here
});

Using ES7 async/await

If you’re coding in an environment that supports async/await, you can also just use a regular for loop and then await a promise in the loop and it will cause the for loop to pause until a promise is resolved before proceeding. This will effectively sequence your async operations so the next one doesn’t start until the previous one is done.

async function processArray(array, fn) {
    let results = [];
    for (let i = 0; i < array.length; i++) {
        let r = await fn(array[i]);
        results.push(r);
    }
    return results;    // will be resolved value of promise
}

// sample usage
processArray(arr, processItem).then(function(result) {
    // all done here
    // array of data here in result
}, function(reason) {
    // rejection happened
});

FYI, I think my processArray() function here is very similar to Promise.map() in the Bluebird promise library which takes an array and a promise producing function and returns a promise that resolves with an array of resolved results.

@vitaly-t – Here some some more detailed comments on your approach. You are welcome to whatever code seems best to you. When I first started using promises, I tended to use promises only for the simplest things they did and write a lot of the logic myself when a more advanced use of promises could do much more of it for me. You use only what you are fully comfortable with and beyond that, you’d rather see your own code that you intimately know. That’s probably human nature.

I will suggest that as I understood more and more of what promises can do for me, I now like to write code that uses more of the advanced features of promises and it seems perfectly natural to me and I feel like I’m building on well tested infrastructure that has lots of useful features. I’d only ask that you keep your mind open as you learn more and more to potentially go that direction. It’s my opinion that it’s a useful and productive direction to migrate as your understanding improves.

Here are some specific points of feedback on your approach:

You create promises in seven places

As a contrast in styles, my code has only two places where I explicitly create a new promise – once in the factory function and once to initialize the .reduce() loop. Everywhere else, I’m just building on the promises already created by chaining to them or returning values within them or just returning them directly. Your code has seven unique places where you’re creating a promise. Now, good coding isn’t a contest to see how few places you can create a promise, but that might point out the difference in leverage the promises that are already created versus testing conditions and creating new promises.

Throw-safety is a very useful feature

Promises are throw-safe. That means that an exception thrown within a promise handler will automatically reject that promise. If you just want the exception to become a rejection, then this is a very useful feature to take advantage of. In fact, you will find that just throwing yourself is a useful way to reject from within a handler without creating yet another promise.

Lots of Promise.resolve() or Promise.reject() is probably an opportunity for simplification

If you see code with lots of Promise.resolve() or Promise.reject() statements, then there are probably opportunities to leverage the existing promises better rather than creating all these new promises.

Cast to a Promise

If you don’t know if something returned a promise, then you can cast it to a promise. The promise library will then do it’s own checks whether it is a promise or not and even whether it’s the kind of promise that matches the promise library you’re using and, if not, wrap it into one. This can save rewriting a lot of this logic yourself.

Contract to Return a Promise

In many cases these days, it’s completely viable to have a contract for a function that may do something asynchronous to return a promise. If the function just wants to do something synchronous, then it can just return a resolved promise. You seem to feel like this is onerous, but it’s definitely the way the wind is blowing and I already write lots of code that requires that and it feels very natural once you get familiar with promises. It abstracts away whether the operation is sync or async and the caller doesn’t have to know or do anything special either way. This is a nice use of promises.

The factory function can be written to create one promise only

The factory function can be written to create one promise only and then resolve or reject it. This style also makes it throw safe so any exception occuring in the factory function automatically becomes a reject. It also makes the contract to always return a promise automatic.

While I realize this factory function is a placeholder function (it doesn’t even do anything async), hopefully you can see the style to consider it:

function factory(idx) {
    // create the promise this way gives you automatic throw-safety
    return new Promise(function(resolve, reject) {
        switch (idx) {
            case 0:
                resolve("one");
                break;
            case 1:
                resolve("two");
                break;
            case 2:
                resolve("three");
                break;
            default:
                resolve(null);
                break;
        }
    });
}

If any of these operations were async, then they could just return their own promises which would automatically chain to the one central promise like this:

function factory(idx) {
    // create the promise this way gives you automatic throw-safety
    return new Promise(function(resolve, reject) {
        switch (idx) {
            case 0:
                resolve($.ajax(...));
            case 1:
                resole($.ajax(...));
            case 2:
                resolve("two");
                break;
            default:
                resolve(null);
                break;
        }
    });
}

Using a reject handler to just return promise.reject(reason) is not needed

When you have this body of code:

    return obj.then(function (data) {
        result.push(data);
        return loop(++idx, result);
    }, function (reason) {
        return promise.reject(reason);
    });

The reject handler is not adding any value. You can instead just do this:

    return obj.then(function (data) {
        result.push(data);
        return loop(++idx, result);
    });

You are already returning the result of obj.then(). If either obj rejects or if anything chained to obj or returned from then .then() handler rejects, then obj will reject. So you don’t need to create a new promise with the reject. The simpler code without the reject handler does the same thing with less code.

Here’s a version in the general architecture of your code that tries to incorporate most of these ideas:

function factory(idx) {
    // create the promise this way gives you automatic throw-safety
    return new Promise(function(resolve, reject) {
        switch (idx) {
            case 0:
                resolve("zero");
                break;
            case 1:
                resolve("one");
                break;
            case 2:
                resolve("two");
                break;
            default:
                // stop further processing
                resolve(null);
                break;
        }
    });
}


// Sequentially resolves dynamic promises returned by a factory;
function sequence(factory) {
    function loop(idx, result) {
        return Promise.resolve(factory(idx)).then(function(val) {
            // if resolved value is not null, then store result and keep going
            if (val !== null) {
                result.push(val);
                // return promise from next call to loop() which will automatically chain
                return loop(++idx, result);
            } else {
                // if we got null, then we're done so return results
                return result;
            }
        });
    }
    return loop(0, []);
}

sequence(factory).then(function(results) {
    log("results: ", results);
}, function(reason) {
    log("rejected: ", reason);
});